WO2000013269A1

WO2000013269A1 - Interferometer based optical devices, particularly amplifiers

Info

Publication number: WO2000013269A1
Application number: PCT/CA1999/000760
Authority: WO
Inventors: Hamid Hatami-Hanza
Original assignee: Opnet Communication Technologies Inc.
Priority date: 1998-08-26
Filing date: 1999-08-18
Publication date: 2000-03-09
Also published as: CA2244160A1; AU5275399A

Abstract

An optical gain module (OGM) in the form of an active Mach-Zehnder interferometer (MZI) is provided. This amplifier module used in various configurations can increase the saturation power by up to twice as much as conventional amplifiers with up to twice the gain, while rejecting up to half of the noise generated in the two arms of the active MZI, thereby decreasing the amplifier noise figure significantly.

Description

INTERFEROMETER BASED OPTICAL DEVICES, PARTICULARLY AMPLIFIERS

Field of Invention

The present invention relates generally to optical devices, and particularly to devices utilizing one or more Mach-Zehnder Interferometers (MZI). More particularly still, it relates to MZI-based optical amplifiers. At the core of the present invention is a basic "building-block" MZI-based optical gain module (OGM), which may be utilized in different optical devices configurations as a modular block.

Background of the Invention

Optical amplifiers are essential components of optical fiber communication systems. They are used as boosters at transmitter end, as preamplifiers at the receiver end, and as optical repeaters in long haul optical transmission lines. They also have other application in other areas, such as sensor systems and a variety of lasers. An optical amplifier usually consists of a gain medium in the form of an optical waveguide doped with appropriate dopants to provide a lasing medium for the signal to be amplified. The gain medium provides the means to transfer energy from an optical pump or electrical current to the to be amplified optical signal.

The performance of an optical amplifier is quantified by its output power delivered at the signal frequency band, noise figure, optical signal gain, and stability and reliability of amplifier operation. The most important problem associated with optical amplifiers is amplified spontaneous emission (ASE) noise in the signal band, generated in the pumped gain media. Appropriate pumping along the fiber can reduce ASE noise and permit a compromise between output power and generated noise in the active media.

To provide a low noise, high gain and high output power optical amplifier, a two stage fiber amplifier has been disclosed in US Patent No. 5,430,572, "High Power, High Gain, Low Noise, Two- Stage Optical Amplifier," issued July 4, 1995 to DiGiovanni et al. The invention uses a low noise-figure first stage amplifier with a shorter length of fiber to achieve a low noise figure and uses another higher gain cascaded amplifier to boost the signal to the desired value. The noise-figure will be determined by the first stage of amplification. 9

3 Another approach used to reduce the amount of noise in the amplified signal has been disclosed in US Patent No. 5,636,053, "Fiber Optic Amplifier System With Noise Figure Reduction," issued June 3, 1997 to Pan. The invention provides a fiber optic amplifier comprising a generic optical amplifier and a controllable polarization beam splitter and optics apparatus to separate half of the noise power from the optically amplified signal. It therefore improves the noise-figure of the amplifier by a maximum 3 dB. However, in practice the noise figure would not be improved significantly unless the amplified signal maintains its polarization. Thus, a polarization maintaining optical active fiber, must be used. The invention also uses free space optical apparatus, such as collimators, and a variable polarization splitter, which can make the amplifier system bulky, expensive, and lack long term reliability.

In US Patent No. 5,757,541, "Method And Apparatus For An Optical Fiber Amplifier," issued May 26, 1998 to Fidric, a simplified and superior optical amplifier configuration has been disclosed. The amplifier comprises an optical circulator, and a 3 dB coupler connected to two opposite ends of the active fiber. The objective of the invention is to use as few as possible optical components. It also provides a more uniform pumping and amplification (by splitting the signal and pump in both directions around the loop) to reduce the noise. 9

4 High power, high gain, low noise, and low cost optical amplifiers are desirable in order to reduce the cost and simplify optical systems. The present invention endeavours to provide optical amplifiers with higher gain and output power, with a lower noise-figure than prior art devices.

SUMMARY OF THE INVENTION

At the core of the present invention an optical gain module (OGM) is provided, which is used in different configurations of optical amplifier systems and other devices.

The OGM is in the form of an active Mach-Zehnder Interferometer (MZI), and comprises a splitter and combiner pair interconnected by two active waveguides as gain media. The two active waveguides have enough separation such that they cannot transfer energy through overlapping of their evanescent fields of their respective guided modes.

The optical signal is first split by the splitter into two parts and is amplified through the optically pumped active arms of the MZI and then recombined at the combiner. The two individual amplified signals, maintaining their relative phase, combine constructively at the desired output port, while the noise, independently generated in the two active waveguides and having random phase relative to each other, will not recombine 9

5 constructively and on average, only half of the total generated noise exits with the amplified signal.

The length of the active waveguides and level of pumping energy can be optimized to achieve a maximum gain and power, with minimum noise generated in each active waveguide. The optical gain module, therefore, provides twice as much saturation power and gain compared to an amplifier using a single active waveguide; while the generated noise remains equal to the noise generated by an amplifier using a single active waveguide with the same length.

This arrangement also can provide bi-directional pumping, which has the advantages of higher gain and lower amplified spontaneous emission (ASE) noise due to a more uniform population inversion of exited states along the individual active waveguide.

In summary, the advantages of the present invention are as follows:

* Up to twice, + 3 dB, higher gain than optical amplifiers using a single active waveguide;

* Up to twice, + 3 dB, higher output power than optical amplifiers using a single active waveguide; 9

6

* Up to - 3 dB noise-figure improvement over conventional or two stage optical amplifiers; and

* The optical amplifier can be implemented in planar optical circuits, making it more suitable for monolithic or hybrid integration, paving the way for further cost reduction with improved reliability.

Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the present invention will now be described in detail by way of example with reference to the accompanying drawings, in which:

Figure 1 shows gain and power saturation characteristic of optical fiber amplifiers versus input signal power at a given pump power;

Figure 2 is a schematic of the preferred embodiment of the optical gain module (OGM) according to the present invention; Figure 3 is a schematic representation of a variation of the preferred embodiment shown in figure 2;

Figure 4 is a schematic of one configuration of an optical amplifier utilizing the OGM of figures 2 or 3;

Figure 5 is a schematic of another configuration of an optical amplifier utilizing the OGM of figures 2 or 3;

Figure 6 is a schematic of another configuration of an optical amplifier utilizing the OGM of figures 2 or 3;

Figure 7 is a schematic block diagram of an optical amplifier, utilizing the OGM of figures 2 or 3, with a control and processing unit;

Figure 8 illustrates an alternative realization of active waveguides shown in the embodiments of figures 2 and 3; and

Figure 9 shows a perspective cross-section of the active waveguides shown in the embodiment of Figure 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The physical mechanism of the optical amplification and its applications have been described by S. Sudo, "Optical fiber ampl ifiers: Materials, Devices and Appl ications/¹ (Artech House, Boston), 1997, and by P.C. Becker et al, "Erbium-Doped Fiber Amplifiers, Fundamentals and Technology," (Academic Press, San Diego), 1999.

Referring now to the drawings, Figure 1 shows the relationship between gain and power in optical fiber amplifiers, where beyond a certain point the output power of an optical amplifier cannot be increased by increasing the input power; that is, the gain decreases for input power beyond the saturation point. As the number of channels in WDM transmission system increases, optical fiber amplifiers with high saturation powers are needed. Increasing the pump power can increase saturation power of an optical amplifier, but the relationship is not linear and efficiency becomes low. That is, the increase in saturation power becomes relatively small compared to the increase in pump power. Hence increasing pump power is not an efficient way to increase saturation power.

Referring now to Figure 2, an optical gain module (OGM) 10 comprises a Mach-Zehnder Interferometer (MZI) having two optical directional couplers 11 and 12 connected by two active waveguides 13 and 14 having ports I, II, III and IV, the first three being input ports, while the fourth being the output of the OGM 10, the signal port of which is port I. The two remaining ports II and III are to receive optical pump signals, which are combined with the input signal to be amplified in the waveguides 13 and 14; therefore eliminating the need for wavelength division multiplexing (WDM) couplers, and above all providing higher gain and lower noise than conventional optical amplifiers.

The basic principle of operation of the OGM 10 is as follows. The first optical waveguide coupler 11 splits the input signal and pump signal almost equally into the two active waveguides 13, 14. The second coupler 12 splits the second pump signal from the opposite side, which is counter-propagating in the active waveguides 13, 14 relative to the signal. The bidirectional pumping improves the population inversion profile along the active waveguides and results in more efficient amplification and generates less noise. The length of the active waveguides 13, 14 and the pump energy can be optimized to achieve the desirable power, or gain, from each individual active waveguide. The optical signal after amplification in the two active arms of the MZI enter the second waveguide coupler 12 and combine constructively at the output port IV. The individually amplified optical signal halves are added constructively because they have phase correlation, i.e. coming from the same source, and conserve their relative phase. The noise, on the other hand, has been generated independently in the two active waveguides 13, 14 having random phase relative to each other, will not add constructively and on average only half 9

10 of the total noise appears at the output port IV. Alternatively, the couplers can be designed such that the signal exits from port III and half of the noise from the port IV, while port IV is also used for counter pumping.

Furthermore, both the first and second couplers 11, 12 can be designed such that when they are cascaded they give a desired coupling function versus wavelength. For instance, the wavelength response of the cascaded first and second couplers 11, 12 can be designed to flatten the optical gain spectrum of the active waveguides 13, 14 yet split the light at pump wavelengths with a desired ratio. In an article by H.A. Haus and N.A. Whitaker, "Elimination of cross talk in optical directional couplers," published in Journal of Appl. Phys. Lett, vol. 46, pp. 1-3, 1985, and in another article by H. Hatami- Hanza and P.L. Chu, "Shaping the switching response of nonlinear directional couplers," published in Optics Com. vol. 119, pp. 347-351, 1995, guidelines are given for achieving a desired spectral response from optical couplers. The desired splitting or coupling ratio at the pump wavelength is 50/50, but slight departures from the desired ratio do not have a significant impact on the operation of the OGM 10.

Due to saturation effects in optical amplifiers at some point, regardless of input signal power, one can only achieve a certain amount of power at the output of a single fiber optical 9

11 amplifier as shown in Figure 1. Therefore, if the input signal is strong enough so that half of the input power can drive an optical amplifier with a single active waveguide to saturation, then both of the active waveguides 13, 14 in the MZI arms will be driven to their maximum output power, i.e. saturation power. But since the MZI configuration adds up the powers at the arms 13, 14, therefore the total saturation power of the MZI-based OGM 10 will be twice as much as an amplifier with a single active waveguide. Moreover, the contaminating noise of the OGM 10 will be equal to the amount of noise generated in only one of the arms 13, 14. Hence, the OGM 10 doubles the saturation power, e.g. gain, without noise penalty.

For satisfactory operation of the OGM 10, the two active waveguides 13, 14 should have exactly the same optical length. Optical length may be tuned, for instance by exposing a section of the fiber in the OGM 10 to Ultra Violet light. Furthermore, both active arms 13, 14 of the OGM 10 should be kept close and sealed in a package so that their ambient conditions, such as temperature, stress and the like are the same at all times. A twin-core Erbium doped fiber can be used to insure that both arms experience similar ambient conditions.

Referring now to Figure 3, another preferred embodiment of the OGM 10 is shown. An MZI 15 having only two ports interconnects two wave-division multiplexers (WDM) 16 and 17, presenting ports 9

12 I, II, III and IV as the OGM's 10 ports. The MZI 15 has the usual active arms, 13 and 14, which are connected two Y-junction splitter/combiners 18 and 19. In this embodiment, the input signal and the optical pump signal are first multiplexed outside the MZI 15 by one of the WDM couplers 16, 17, which applies them to the MZI 15. Again, bi-directional optical pumping may be used here. The manner of operation is the same as the OGM 10 in Figure 2, and the saturation power, and gain, will be twice that of a single active waveguide amplifier. The same observation, made in connexion with the OGM 10 of Figure 2 apply here as well.

Following is a brief exposition of how signal-to-noise ratio (SNR) is improved in an amplifier using the present OGM. A commonly used quantity for measuring the noise performance of an amplifier is its noise figure (NF) defined as:

NF (dB) = SNR_0Utput (dB) - SNR_tnpϋt (dB) where

SNR_0Utput (dB) log₁₀ (signal power at output/noise power at output) and SNR_iπput (dB) log₁₀ (signal power at input/noise power at input).

A detailed discussion of noise characteristics and analysis in optical amplifiers has been given by P.C. Becker et al in "Erbium-doped Fiber Amplifiers, Fundamentals and Technology," 9

13 (Academic Press, San Diego), Chapter 7, 1999. The present OGM contaminates the output-amplified signal by half the amount of noise generated in the module. Hence, the NF of the amplifier using the OGM 10 is reduced by 3 dB. As was shown above, the saturation power of the OGM 10 is twice that of a conventional optical gain module with single active waveguide. The noise of an amplifier configuration using the OGM 10 will be 3 dB lower than other conventional amplifiers working at saturation and, theoretically, could approach zero dB.

The OGM 10 may be placed and sealed in a temperature controlled box where it is hermetically sealed inside the box by a heat conductive adhesive such as ceramics or any other suitable material. When adjusting the optical length and phase of the signal in one of the arms of an MZI, the UV tuning operation may be done at the final stages of testing of the OGM to ensure the required phase relationship of the signals in the two active waveguides 13, 14. The present OGM is particularly suitable for integrated optics. It will operate best when fabricated using planar waveguide technologies such as ion exchange, silica on silicon, or doped sol-gel glass. Ion exchanged active waveguide devices are given, for instance, in the paper by Najafi et al. entitled, "Ion-exchanged rare-earth doped waveguides," published in SPIE vol. 1128 Glasses for Optoelectronics, 1989, pp. 142- 144. 9

14

Referring now to Figure 4, it shows the OGM 10 connected in one implementation of an optical amplifier system. The input signal is applied to Port I via an Optical Isolator (10) 20, while Port IV supplied the output signal via 01 21. Ports II and III of the OGM 10 receive pump signals from optical pumps (OP) I and

II, respectively. 01s 20 and 21 ensure unidirectionality in the directions shown by the arrows and don't interfere with the operation of the OGM 10 as described above.

In Figure 5, a similar amplifier system to that of Figure 4 is shown except that the OPs I and II are combined in 3-dB coupler

22, the outputs of which are applied in the OGM 10 ports II and

III. In such configuration, both OPs contribute to both jumping directors. Alternatively, only one OP may be coupled to the 3- dB coupler 22 to provide bi-directional pumping. The second OP is then optional and may be used when there is need for higher power or for provision of a back up pump should the first fail, thus improving reliability.

In Figure 6 a two stage optical amplifier is shown using the OGM 10 as implemented in Figure 4 or 5. The first stage is a low- noise low-gain single waveguide amplifier stage preferable with a noise figure of less than 2 dB followed by the high-power high gain OGM. This arrangement provides very high gain, high- saturation power and very low noise figure optical amplifier system. The first stage is to provide enough gain to compensate 9

15 for the 3-dB decrease in the divided input signal, and comprises a WDM 23 applying the input signal and pump signal from OP 24 to an active waveguide 25.

In Figure 7 there is provided an amplifier system similar to that shown in Figure 5, but further having means for monitoring and controlling the operation of the optical amplifier system. Two optical taps 26 and 27 direct a fraction of the energy of the input signal and the amplified output signal to a processing and control unit 28. The processing and control unit 28 comprises means for filtering samples of the optical signals at different bands, such as supervisory channel information signals, detects the sampled signals and transform them to corresponding electrical signals. The processing unit 28 further processes and analyzes different properties of the detected signals and calculates different parameters of the optical amplifier such as gain, noise, output power and the like. The processing unit 28 may also measure the ambient temperature, send control signals to the optical source(s) to control the optical pump power or to adjust the temperature of OGM 10 to ensure satisfactory operation of the optical amplifier.

In Figure 8, an alternative realization of the active waveguides 13 and 14 of figures 2 and 3 is shown. Such alternative obviates the use of external optical pumps such as optical pumps I and II in Figures 4 and 5, because the active waveguides 13' and 14' are semiconductor waveguides, which are pumped by carrier injection. A cross-section of such a semiconductor waveguide is shown in perspective in Figure 9, wherein a top and bottom conductor layers 29 and 30 are deposited at the desired waveguide positions and "sandwich" an upper cladding layer 31 underneath the top conductor 29 and a lower buffer layer 32 on top of the lower electrode 30, while an active core layer 33 is located between the cladding 31 and buffer 32. In operation an electric current is injected via conductor 34 between the electrodes 29 and 30, which pump the energy to the active layer 33. An advantage of such active waveguide realization is that input signal amplification may be accomplished at desired wavelengths by band-gap design of the semiconductor to be in a selected region. The principle of operation of such semiconductor amplifier has been described, for instance, in the book entitled, "Introduction to Semiconductor Integrated Optics", Chapter 9, by Hans P. Zappe, Artech House, Boston, Massachussets, U.S.A., 1995.

Claims

917 WHAT IS CLAIMED IS:

1. An optical device for providing optical gain, comprising:

(a) an optical interferometer having two arms, an optical input and an optical output; and

(b) at least one arm of said interferometer being an optically active arm.

2. The optical device as defined in Claim 1, said two arms being optically active when optically pumped.

3. The optical device as defined in Claim 1 said interferometer comprising: two un-coupled identical active optical waveguides interconnecting a pair of four-port optical directional couplers; and unconnected two of the four-ports of the directional couplers constituting input and output ports of an optical gain module (OGM).

4. The optical device as defined in Claim 1 said interferometer comprising: two un-coupled identical active optical waveguides interconnecting a pair of three port optical Y-junctions; and unconnected ports of the Y-junctions constituting input and output ports of said interferometer.

5. The optical device as defined in claim 3, said interferometer being a Mach-Zehnder Interferometer (MZI).

6. The optical device as defined in claim 4, said interferometer being a Mach-Zehnder Interferometer (MZI).

7. The optical device as defined in claim 6, said input and output ports of the MZI interconnecting a pair of three-port wave-division multiplexers (WDM); and unconnected two of the three-ports of the WDMs constituting input and output ports of an optical gain module (OGM).

8. The optical device as defined in claim 7, said OGM having four-ports, the input and output ports, and at least one of the remaining two ports connected to an optical pump.

9. The optical device as defined in claim 5, said OGM having four-ports, the input and output ports, and at least one of the remaining two port, connected to an optical pump.

10. The optical device as defined in claim 1, said optically active arm being a rare-earth ion doped optical waveguide.

11. The optical device as defined in claim 10, said rare-earth ion being erbium.

12. The optical device as defined in claim 1, said optically active arm being a semiconductor waveguide.

13. The optical device as described in claim 12, said semiconductor waveguide being activated by injection of electrical current.

14. The optical device as defined in claim 13, said interferometer being a Mach-Zehnder Interferometer (MZI) having two active arms interconnecting first and second three-port Y- junctions, the first Y-junction operating as a splitter of an input optical signal thereto, and the second Y-junction operating as a combiner providing an output optical signal.

15. The optical device as defined in claim 1, said two arms of the optical interferometer being two optical waveguides of a twin-core optical fiber.

16. The optical device as defined in claim 5, said two arms of the optical interferometer being two optical waveguides of a twin-core optical fiber.

17. The optical device as defined in claim 6, said two arms of the optical interferometer being two optical waveguides of a twin-core optical fiber.

18. The optical device as defined in claim 7, said two arms of the optical interferometer being two optical waveguides of a twin-core optical fiber.

19. The optical device as defined in claim 8, said two arms of the optical interferometer being two optical waveguides of a twin-core optical fiber.

20. The optical device as defined in claim 9, said two arms of the optical interferometer being two optical waveguides of a twin-core optical fiber.