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WO2003089970A2 - A wavelength monitor using an arrayed waveguide grating - Google Patents

A wavelength monitor using an arrayed waveguide grating Download PDF

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Publication number
WO2003089970A2
WO2003089970A2 PCT/GB2003/001547 GB0301547W WO03089970A2 WO 2003089970 A2 WO2003089970 A2 WO 2003089970A2 GB 0301547 W GB0301547 W GB 0301547W WO 03089970 A2 WO03089970 A2 WO 03089970A2
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WO
WIPO (PCT)
Prior art keywords
input
waveguides
free space
output
peaks
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PCT/GB2003/001547
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French (fr)
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WO2003089970A3 (en
Inventor
Mark Volanthen
Richard Laming
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Avanex Uk Limited
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Publication date
Application filed by Avanex Uk Limited filed Critical Avanex Uk Limited
Publication of WO2003089970A2 publication Critical patent/WO2003089970A2/en
Publication of WO2003089970A3 publication Critical patent/WO2003089970A3/en

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
    • H04B10/075—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal
    • H04B10/079—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal using measurements of the data signal
    • H04B10/0795—Performance monitoring; Measurement of transmission parameters
    • H04B10/07955—Monitoring or measuring power
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B6/12007—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer
    • G02B6/12009—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides
    • G02B6/12016—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides characterised by the input or output waveguides, e.g. tapered waveguide ends, coupled together pairs of output waveguides
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B6/12007—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer
    • G02B6/12009—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides
    • G02B6/12019—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides characterised by the optical interconnection to or from the AWG devices, e.g. integration or coupling with lasers or photodiodes
    • G02B6/12021—Comprising cascaded AWG devices; AWG multipass configuration; Plural AWG devices integrated on a single chip
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B6/12007—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer
    • G02B6/12009—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides
    • G02B6/12033—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides characterised by means for configuring the device, e.g. moveable element for wavelength tuning
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
    • H04B10/075—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal
    • H04B10/077—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal using a supervisory or additional signal

Definitions

  • the present invention relates to a tuneable optical receiver. More particularly, but not exclusively, the present invention relates to a tuneable optical receiver including an arrayed waveguide grating (AWG) and a plurality of input and output waveguides arranged to enable the simultaneous measurement of power at a plurality of signal frequencies.
  • AMG arrayed waveguide grating
  • Optical waveguides often simultaneously carry a plurality of signals of different frequencies. It can be useful to be able to measure these signals arid also the noise of frequencies between the signals, and thus obtain the signal-to-noise ratio (SNR).
  • SNR signal-to-noise ratio
  • Power monitors based on continuously tuneable filters are known.
  • the filter is scanned across a predetermined wavelength/frequency range whilst a detector makes regular measurements.
  • Such power monitors can provide an accurate measurement of the signal and signal to noise ratio across a frequency/wavelength range.
  • the measurements are time consuming and are not simultaneous (for different wavelengths/frequencies).
  • such scanning filters are too slow for safety cut-out circuits for use in, for example, network protection systems, which are commonly based on a feedback switch which detects unacceptable SNR's.
  • Such feedback switches are typically required to have a response time of 5 milliseconds or less.
  • Power monitors comprising a diffraction grating and a large array of detectors, the array having more detectors than the number of output channels, are also known.
  • detectors can make simultaneous measurements at a plurality of wavelengths/frequencies but are bulky and difficult to manufacture and maintain.
  • the present invention provides a tuneable optical receiver comprising:-
  • an arrayed waveguide grating comprising first and second free space regions and a plurality of array waveguides optically coupled therebetween; M input waveguides optically coupled to the first free space region, where M>1;
  • N output waveguides optically coupled to the second free space region, each output waveguide being in optical communication with an optical detector;
  • the transmission function T m ( ⁇ ) of the tuneable optical receiver the for m'th input waveguide where 0 ⁇ m ⁇ M, comprising a series of peaks in transmission as a function of wavelength;
  • the input and output waveguides are arranged such that at least some of the peaks of the transmission function T m ( ⁇ ) corresponding to the m'th input waveguide are interleaved with at least some of the peaks of the respective transmission functions corresponding to the other input waveguides.
  • the receiver according to the invention can make simultaneous optical measurements at N wavelengths/frequencies.
  • simultaneous optical measurements can then also be made of a range of other frequencies interleaved with those of the original frequencies, without any need to increase the number of detectors. These measurements can then be used to monitor noise between the original frequencies.
  • the optical detectors are power detectors. This enables a receiver to provide parallel power measurements and thus measurements of signal to noise ratio can be obtained.
  • the tuneable receiver further comprises a switching means optically connected to a signal input waveguide, the switching means also being connected to the M input waveguides and being adapted to switch an optical signal from the signal input waveguide to one or more of the M input waveguides.
  • a switching means optically connected to a signal input waveguide
  • the switching means also being connected to the M input waveguides and being adapted to switch an optical signal from the signal input waveguide to one or more of the M input waveguides.
  • the input and output waveguides are arranged such that the spacing between the peaks in the transmission function T m ( ⁇ ) corresponding to the m'th input waveguide is a non-integer multiple of the spacing between the peaks in the transmission function T n ( ⁇ ) corresponding to at least one other of the input waveguides. This ensures that by switching the input signal between the input waveguides a different range of wavelengths is presented at the output waveguides, so increasing the effective resolution of the receiver, without increasing the number of detectors.
  • Figure 1 shows the signal ampitude as a function of frequency for an optical waveguide
  • Figure 2 shows in a schematic form, a known power monitor including a continuously tuneable filter
  • Figure 3 shows in a schematic form an arrayed waveguide grating demultiplexer
  • Figure 4 shows the transmission function of the arrayed waveguide grating demultiplexer of Figure 3;
  • Figure 5 shows a tuneable optical receiver according to the invention in schematic form
  • Figure 6 shows the transmission function of the arrayed waveguide grating demultiplexer of the optical receiver of Figure 5
  • Figures 7(a) and Figure 7(b) show in a schematic form the operation of the optical receiver of Figure 5
  • Shown in Figure 1 is a typical signal ampitude vs. frequency plot for an optical waveguide being used for example to carry voice or video signals.
  • the waveguide carries several signals at different frequencies.
  • FIG. 2 Shown in Figure 2 is a known-power monitor comprising a continuously tuneable filter 2 which is optically connected to the optical waveguide 1.
  • a continuously tuneable filter 2 which is optically connected to the optical waveguide 1.
  • the signal which passes through the filter 2 is detected by an optical power detector 3 and may (if desired) be recorded by computer 4 and/or fed into another circuit.
  • the filter slowly scans across the selected frequency range building up a picture of the signals carried by the optical waveguide
  • Such a power monitor is slow and does not make simultaneous measurements.
  • FIG. 3 Shown in Figure 3 is a power detector including an arrayed waveguide grating (AWG) demultiplexer.
  • the demultiplexer comprises a substrate or "die” 10 having provided thereon an input waveguide 11 for a multiplexed input signal, two free space regions 12, 13 in the form of a "slab” waveguides connected to either end of an arrayed waveguide grating consisting of an array of transmission waveguides 14, only some of which are shown, a plurality of output waveguides 15 (only some shown) for outputting respective wavelength channel outputs from the second (output) slab waveguide 13 to the edge 16 of the die.
  • AMG arrayed waveguide grating
  • the present inventive can also be applied to devices formed using other types of waveguides, for example, ridge waveguides.
  • Shown in Figure 4 is the transmission function T( ⁇ ) of the arrayed waveguide grating demultiplexer of Figure 3.
  • the transmission function is plotted in Figure 4 as signal amplitude is a function of wavelength, receiver by the output waveguides.
  • the transmission function in the wavelength range shown comprises a plurality of substantially Gaussian peaks, each peak being produced by a separate output waveguide.
  • By optically complying a respective power detector to each of the output waveguides (as shown in broken lines in Figure 3) it is possible to measure power in the input signal, each at a different wavelength.
  • the power monitor so formed can therefore be used to simultaneously measure power at a range of wavelengths/frequencies.
  • the number of simultaneous measurements which can be made is limited by the number of output waveguides and power detectors connected thereto.
  • the optical receiver comprises an arrayed waveguide grating demultiplexer of a structure similar to that in Figure 3.
  • the demultiplexer however comprises a plurality of input waveguides 30 optically connected to the first slab waveguide 12. These input waveguides are in turn optically connected to a switching means 25 which is adapted to switch an input signal, from a signal input lightguide in the form of (in this case) an optical fibre 35) between the optical input waveguides.
  • the arrayed waveguide grating demultiplexer shown in Figure 5 has M input waveguide 30 numbered 1 ...M respectively. Similarly, the arrayed waveguide grating has N output waveguides 15 numbered 1....N respectively.
  • the transmission function T m ( ⁇ ) of this arrayed demultiplexer of Figure 5 where the m'th input waveguide is used (hereinafter referred to as all transmission function T m ( ⁇ ) corresponding to the m'th input waveguide) comprises a series of peaks similar to that shown in Figure 4, the peaks being substantially equally spaced in terms of wavelength (or frequency). Since the arrayed waveguide grating has M input waveguides the AWG will have M corresponding transmission functions / T ⁇ ( ⁇ ) to T M ( ⁇ ).
  • the input waveguides are spaced at the input face of the first slab waveguide such that the peaks in the transmission functions are interleaved (along the wavelength axis) as shown in Figure 6.
  • the peaks of the transmission functions are separated from each other in wavelength, the respective peaks of each transmission function being arranged to lie between (and interspersed with) the peaks of the other transmission functions.
  • a respective optical power detector 20 is provided in optical communication with an output end of each of the N output waveguides, as shown in Figure 5.
  • FIG. 7(a) Shown in Figure 7(a) is a schematic of the arrayed waveguide grating receiving a multi frequency signal F via the m'th input waveguide. This is de-multiplexed by the arrayed waveguide grating and the amplitudes of the signal F at wavelengths ⁇ i, ⁇ 2 and ⁇ are received by the output waveguides. The signal is then transmitted to the power detectors 20 which measure the power at these wavelengths. These power measurements can be recorded if desired, or may simply be monitored continuously by a monitoring circuit 38.
  • the input signal is switched from the m'th waveguide to a different input waveguide as shown in Figure 7(b). Again, the AWG demultiplexes the signal and this is again received by the output waveguides.
  • Changing the input signal from one input waveguide to a different input waveguide shifts the transmission function slightly (along the wavelength axis) as shown in Figure 6. Since the peaks in the M transmission functions are all interleaved the signals received at the output waveguide no longer correspond to the ampitude of the signal F at wavelengths ⁇ i, ⁇ 2 and ⁇ but instead now correspond to the amplitude of the signal F at wavelengths ⁇ + ⁇ , ⁇ 2 + ⁇ , and ⁇ + ⁇ . Accordingly, with the optical receiver according to the invention one can simultaneously measure the signal of N wavelengths and then by changing the input waveguide one can simultaneously measure at a further set of N different wavelengths, without the need for additional detectors.
  • the input waveguides are all equally spaced (physically) at the input face of the first slab waveguide 12, but this need not always be the case.
  • the physical spacing of the input waveguides, where they are coupled to the input face of the first slab waveguide, determines the relative spacing of the peaks of the different transmission functions corresponding to the different input waveguides.
  • the physical spacing(s) of the input waveguides can be designed so that the spacing of the peaks in each transmission function T m ( ⁇ ) is a non-integer multiple of the spacing of the peaks in each of the other transmission functions. In this manner, the peaks in each transmission function T m ( ⁇ ) will be interleaved with the peaks of the other transmission functions.
  • the input fibre 35 carrying the input signal may be scanned across the input face 42 of the first slab waveguide 12 between M predetermined positions at which power measurements are desired to be made.
  • the switching means 25 which could for example be in the form of an electrically-controlled mechanical switch
  • the input fibre 35 carrying the input signal may be scanned across the input face 42 of the first slab waveguide 12 between M predetermined positions at which power measurements are desired to be made.
  • the first slab waveguide 12 is located at the edge 42 of the die 10 on which the AWG is fabricated so that at least a portion of the input face 18 of the first slab waveguide is flush with the (input) edge face 42 of the die.
  • the end of the input fibre is mounted to a movable mechanical platform 40 which is automatically controlled to move (preferably in incremental fashion) the platform, and consequently the input fibre end, up and/or down the input edge 42 of the die between the M predetermined positions, in a direction pe ⁇ endicular, or at least substantially pe ⁇ endicular, to the optical axis X of the slab waveguide.
  • the fibre end can be scanned across the input face of the slab waveguide, between the various predetermined positions at which positions power measurements are made by the detectors at the output ends of the output waveguides.
  • the positions may, for example, be chosen so as to enable SNR to be calculated (this may for example be done automatically by SNR calculation software provided in, or operating in conjunction with, the driving electronics for the power detectors.
  • there are no predetermined positions for the signal input fibre 35 instead the fibre end is scanned incrementally across the input face of the slab waveguide while power measurements are continuously made by the detectors 20 at the output ends of the output waveguides 15.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Electromagnetism (AREA)
  • Computer Networks & Wireless Communication (AREA)
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Abstract

An arrayed waveguide grating comprising first (12) and second (13) free space regions and a plurality of array waveguides (14) optically coupled therebetween; M input waveguides (30) optically coupled to the first free space region, where M>1; and N output waveguides (15) optically coupled to the second free space region, each output waveguide being in optical communication with an optical detector (20), the transmission function Tm(λ) of the tuneable optical receiver, for the m'th input waveguide where 0<m≤M, comprising a series of peaks in transmission as a function of wavelength; wherein the input and output waveguides are arranged such that at least some of the peaks of the transmission function Tm(λ) corresponding to the m'th input waveguide are interleaved with at least some of the peaks of the respective transmission functions corresponding to the other input waveguides.

Description

A TUNEABLE OPTICAL RECEIVER
The present invention relates to a tuneable optical receiver. More particularly, but not exclusively, the present invention relates to a tuneable optical receiver including an arrayed waveguide grating (AWG) and a plurality of input and output waveguides arranged to enable the simultaneous measurement of power at a plurality of signal frequencies.
Optical waveguides often simultaneously carry a plurality of signals of different frequencies. It can be useful to be able to measure these signals arid also the noise of frequencies between the signals, and thus obtain the signal-to-noise ratio (SNR).
Power monitors based on continuously tuneable filters are known. The filter is scanned across a predetermined wavelength/frequency range whilst a detector makes regular measurements. Such power monitors can provide an accurate measurement of the signal and signal to noise ratio across a frequency/wavelength range. However, the measurements are time consuming and are not simultaneous (for different wavelengths/frequencies). In particular, such scanning filters are too slow for safety cut-out circuits for use in, for example, network protection systems, which are commonly based on a feedback switch which detects unacceptable SNR's. Such feedback switches are typically required to have a response time of 5 milliseconds or less.
Power monitors comprising a diffraction grating and a large array of detectors, the array having more detectors than the number of output channels, are also known. Such detectors can make simultaneous measurements at a plurality of wavelengths/frequencies but are bulky and difficult to manufacture and maintain.
Accordingly, in a first aspect, the present invention provides a tuneable optical receiver comprising:-
an arrayed waveguide grating comprising first and second free space regions and a plurality of array waveguides optically coupled therebetween; M input waveguides optically coupled to the first free space region, where M>1; and
N output waveguides optically coupled to the second free space region, each output waveguide being in optical communication with an optical detector;
the transmission function Tm(λ) of the tuneable optical receiver, the for m'th input waveguide where 0 <m<M, comprising a series of peaks in transmission as a function of wavelength;
wherein the input and output waveguides are arranged such that at least some of the peaks of the transmission function Tm(λ) corresponding to the m'th input waveguide are interleaved with at least some of the peaks of the respective transmission functions corresponding to the other input waveguides.
The receiver according to the invention can make simultaneous optical measurements at N wavelengths/frequencies. Importantly, because of the way the input and output waveguides are arranged, by switching the signal between the M input waveguides simultaneous optical measurements can then also be made of a range of other frequencies interleaved with those of the original frequencies, without any need to increase the number of detectors. These measurements can then be used to monitor noise between the original frequencies.
Preferably, the optical detectors are power detectors. This enables a receiver to provide parallel power measurements and thus measurements of signal to noise ratio can be obtained.
Preferably, the tuneable receiver further comprises a switching means optically connected to a signal input waveguide, the switching means also being connected to the M input waveguides and being adapted to switch an optical signal from the signal input waveguide to one or more of the M input waveguides. This provides a simple method for switching the input feed signal between the M input waveguides. Preferably, the input and output waveguides are arranged such that the spacing between the peaks in the transmission function Tm(λ) corresponding to the m'th input waveguide is a non-integer multiple of the spacing between the peaks in the transmission function Tn(λ) corresponding to at least one other of the input waveguides. This ensures that by switching the input signal between the input waveguides a different range of wavelengths is presented at the output waveguides, so increasing the effective resolution of the receiver, without increasing the number of detectors.
The present invention will now be described by way of example only and not in any limitative sense with reference to the accompanying drawings in which
Figure 1 shows the signal ampitude as a function of frequency for an optical waveguide;
Figure 2 shows in a schematic form, a known power monitor including a continuously tuneable filter;
Figure 3 shows in a schematic form an arrayed waveguide grating demultiplexer;
Figure 4 shows the transmission function of the arrayed waveguide grating demultiplexer of Figure 3;
Figure 5 shows a tuneable optical receiver according to the invention in schematic form;
Figure 6 shows the transmission function of the arrayed waveguide grating demultiplexer of the optical receiver of Figure 5
Figures 7(a) and Figure 7(b) show in a schematic form the operation of the optical receiver of Figure 5 Shown in Figure 1 is a typical signal ampitude vs. frequency plot for an optical waveguide being used for example to carry voice or video signals. As can be seen the waveguide carries several signals at different frequencies. In addition there is a small amount of wideband background noise.
It is often useful for engineers to be able to measure the ampitude of both the signals and the noise carried by the waveguide. Shown in Figure 2 is a known-power monitor comprising a continuously tuneable filter 2 which is optically connected to the optical waveguide 1. In use the signal which passes through the filter 2 is detected by an optical power detector 3 and may (if desired) be recorded by computer 4 and/or fed into another circuit. The filter slowly scans across the selected frequency range building up a picture of the signals carried by the optical waveguide Such a power monitor is slow and does not make simultaneous measurements.
Shown in Figure 3 is a power detector including an arrayed waveguide grating (AWG) demultiplexer. The demultiplexer comprises a substrate or "die" 10 having provided thereon an input waveguide 11 for a multiplexed input signal, two free space regions 12, 13 in the form of a "slab" waveguides connected to either end of an arrayed waveguide grating consisting of an array of transmission waveguides 14, only some of which are shown, a plurality of output waveguides 15 (only some shown) for outputting respective wavelength channel outputs from the second (output) slab waveguide 13 to the edge 16 of the die. In a generally known manner there is a constant pre-determined optical path length difference between the lengths of adjacent waveguides 14 in the array which are arranged such that the multiplexed signal input at the input face 18 of the first slab waveguide is demultiplexed into different wavelength output channel signals which are focussed onto the output face 19 of the second slab waveguide. The waveguides are typically high index cores on the substrate of the die. The cores are covered by low index cladding. However, the present inventive can also be applied to devices formed using other types of waveguides, for example, ridge waveguides.
Shown in Figure 4 is the transmission function T(λ) of the arrayed waveguide grating demultiplexer of Figure 3. The transmission function is plotted in Figure 4 as signal amplitude is a function of wavelength, receiver by the output waveguides. The transmission function in the wavelength range shown comprises a plurality of substantially Gaussian peaks, each peak being produced by a separate output waveguide. By optically complying a respective power detector to each of the output waveguides (as shown in broken lines in Figure 3) it is possible to measure power in the input signal, each at a different wavelength. The power monitor so formed can therefore be used to simultaneously measure power at a range of wavelengths/frequencies. However, the number of simultaneous measurements which can be made is limited by the number of output waveguides and power detectors connected thereto.
Shown in Figure 5 is an optical receiver according to the invention in this embodiment being used as a power monitor. The optical receiver comprises an arrayed waveguide grating demultiplexer of a structure similar to that in Figure 3. The demultiplexer however comprises a plurality of input waveguides 30 optically connected to the first slab waveguide 12. These input waveguides are in turn optically connected to a switching means 25 which is adapted to switch an input signal, from a signal input lightguide in the form of (in this case) an optical fibre 35) between the optical input waveguides.
The arrayed waveguide grating demultiplexer shown in Figure 5 has M input waveguide 30 numbered 1 ...M respectively. Similarly, the arrayed waveguide grating has N output waveguides 15 numbered 1....N respectively. The transmission function Tm(λ) of this arrayed demultiplexer of Figure 5 where the m'th input waveguide is used (hereinafter referred to as all transmission function Tm(λ) corresponding to the m'th input waveguide) comprises a series of peaks similar to that shown in Figure 4, the peaks being substantially equally spaced in terms of wavelength (or frequency). Since the arrayed waveguide grating has M input waveguides the AWG will have M corresponding transmission functions/ Tι(λ) to TM(λ). The input waveguides are spaced at the input face of the first slab waveguide such that the peaks in the transmission functions are interleaved (along the wavelength axis) as shown in Figure 6. The peaks of the transmission functions are separated from each other in wavelength, the respective peaks of each transmission function being arranged to lie between (and interspersed with) the peaks of the other transmission functions. A respective optical power detector 20 is provided in optical communication with an output end of each of the N output waveguides, as shown in Figure 5.
The operation of the power monitor of Figure 5 can be explained with reference to schematic Figures 7(a) and 7(b). Shown in Figure 7(a) is a schematic of the arrayed waveguide grating receiving a multi frequency signal F via the m'th input waveguide. This is de-multiplexed by the arrayed waveguide grating and the amplitudes of the signal F at wavelengths λi, λ2 and λ are received by the output waveguides. The signal is then transmitted to the power detectors 20 which measure the power at these wavelengths. These power measurements can be recorded if desired, or may simply be monitored continuously by a monitoring circuit 38.
In order to measure the power in signal F at a different set of wavelengths the input signal is switched from the m'th waveguide to a different input waveguide as shown in Figure 7(b). Again, the AWG demultiplexes the signal and this is again received by the output waveguides. Changing the input signal from one input waveguide to a different input waveguide shifts the transmission function slightly (along the wavelength axis) as shown in Figure 6. Since the peaks in the M transmission functions are all interleaved the signals received at the output waveguide no longer correspond to the ampitude of the signal F at wavelengths λi, λ2 and λ but instead now correspond to the amplitude of the signal F at wavelengths λι+ Δ, λ2 +Δ, and λ +Δ. Accordingly, with the optical receiver according to the invention one can simultaneously measure the signal of N wavelengths and then by changing the input waveguide one can simultaneously measure at a further set of N different wavelengths, without the need for additional detectors.
In the embodiment of Figure 5 the input waveguides are all equally spaced (physically) at the input face of the first slab waveguide 12, but this need not always be the case. The physical spacing of the input waveguides, where they are coupled to the input face of the first slab waveguide, determines the relative spacing of the peaks of the different transmission functions corresponding to the different input waveguides. Based on the known design of the AWG, for any given physical spacing(s) of the output waveguides (at the output face of the second slab waveguide 13), the physical spacing(s) of the input waveguides can be designed so that the spacing of the peaks in each transmission function Tm(λ) is a non-integer multiple of the spacing of the peaks in each of the other transmission functions. In this manner, the peaks in each transmission function Tm(λ) will be interleaved with the peaks of the other transmission functions.
The operation of the invention has been explained with reference to AWG demultiplexers with transmission functions having Gaussian peaks. Nevertheless, array waveguide grating demultiplexers incorporating peak flattening features (also known as "passband flattening" features) are known and may alternatively be used.
Further modifications to the above embodiments are possible within the scope of the invention. For example, instead of the switching means 25, which could for example be in the form of an electrically-controlled mechanical switch, the input fibre 35 carrying the input signal may be scanned across the input face 42 of the first slab waveguide 12 between M predetermined positions at which power measurements are desired to be made. In such an embodiment, illustrated in Figure 8 (like parts to the Figure 5 embodiment are referenced by like reference numerals), there is no need for any input waveguides 30 coupled to the first slab waveguide; instead the first slab waveguide 12 is located at the edge 42 of the die 10 on which the AWG is fabricated so that at least a portion of the input face 18 of the first slab waveguide is flush with the (input) edge face 42 of the die. The end of the input fibre is mounted to a movable mechanical platform 40 which is automatically controlled to move (preferably in incremental fashion) the platform, and consequently the input fibre end, up and/or down the input edge 42 of the die between the M predetermined positions, in a direction peφendicular, or at least substantially peφendicular, to the optical axis X of the slab waveguide. In this manner the fibre end can be scanned across the input face of the slab waveguide, between the various predetermined positions at which positions power measurements are made by the detectors at the output ends of the output waveguides. The positions may, for example, be chosen so as to enable SNR to be calculated (this may for example be done automatically by SNR calculation software provided in, or operating in conjunction with, the driving electronics for the power detectors. In another possible embodiment, there are no predetermined positions for the signal input fibre 35: instead the fibre end is scanned incrementally across the input face of the slab waveguide while power measurements are continuously made by the detectors 20 at the output ends of the output waveguides 15.

Claims

1. A tuneable optical receiver comprising:
an arrayed waveguide grating comprising first and second free space regions and a plurality of array waveguides optically coupled therebetween;
M input waveguides optically coupled to the first free space region, where M>1; and
N output waveguides optically coupled to the second free space region, each output waveguide being in optical communication with an optical detector; the transmission function Tm(λ) of the tuneable optical receiver, for the m'th input waveguide where 0<m<M, comprising a series of peaks in transmission as a function of wavelength; wherein the input and output waveguides are arranged such that at least some of the peaks of the transmission function Tm(λ) corresponding to the m'th input waveguide are interleaved with at least some of the peaks of the respective transmission functions corresponding to the other input waveguides.
2. A tuneable optical receiver as claimed in claim 1, wherein the optical detectors are power detectors.
3. A tuneable optical receiver as claimed in either of claims 1 and 2, further comprising a switching means optically connected to a signal input lightguide, the switching means also being connected to the M input waveguides and being adapted to switch an optical signal from the signal input lightguide to one or more of the M input waveguides.
4. A tuneable optical receiver as claimed in claim 4, wherein the input waveguides and output waveguides are arranged such that the spacing between the peaks in the transmission function Tm(λ) corresponding to the m'th input waveguide is a non-integer multiple of the spacing between the peaks in the transmission function Tn(λ) corresponding to at least one other of the input waveguides.
5. A tuneable optical receiver comprising an arrayed waveguide grating comprising first and second free space regions and a plurality of array waveguides optically coupled therebetween; and
N output waveguides optically coupled to the second free space region, each output waveguide being in optical communication with an optical detector; an adjustable signal input lightguide having an output end thereof optically coupled to an input face of the first free space region, said output end of the input lightguide being mounted on moveable mounting means; and automatic control means for moving said output end of the signal input lightguide between M different predetermined input positions, the transmission function Tm(λ) of the tuneable optical receiver, for the m'th said input position of the fibre end, where 0<m<M, comprising a series of peaks in transmission as a function of wavelength; wherein said predetermined input positions and the output waveguides are arranged such that at least some of the peaks of the transmission function Tm(λ) corresponding to the m'th input position are interleaved with at least some of the peaks of the respective transmission functions corresponding to the other predetermined input positions of the lightguide end.
6. A tuneable optical receiver according to claim 5, wherein the output end of the signal input lightguide is adjacent to the input face of the first free space region.
7. A tuneable optical receiver according to claim 6, wherein the control means is configured to move the output end of the signal input lightguide incrementally between adjacent ones of said M predetermined positions in a direction substantially peφendicular to an optical axis of the first free space region, so as to scan said output end of the signal input lightguide across the input face of the first free space region.
8. A tuneable receiver according to any of claims 5 to 7, wherein the optical detectors are power detectors.
9. A power monitor comprising: an arrayed waveguide grating comprising first and second free space regions and a plurality of array waveguides optically coupled therebetween; and
N output waveguides optically coupled to the second free space region, each output waveguide being in optical communication with an optical power detector; a signal input lightguide having an output end thereof disposed adjacent to an input face of the first free space region, said output end of the input lightguide fibre being mounted on movable mounting means; and automatic control means for moving said moveable mounting means in a direction substantially peφendicular to an optical axis of the first free space region so as to scan said output end of the signal input lightguide across the input face of the first free space region.
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