WO2001067561A2

WO2001067561A2 - Optical waveguide amplifier

Info

Publication number: WO2001067561A2
Application number: PCT/US2001/006726
Authority: WO
Inventors: Brian L. Lawrence
Original assignee: Molecular Optoelectronics Corporation
Priority date: 2000-03-03
Filing date: 2001-03-02
Publication date: 2001-09-13
Also published as: WO2001067561A3; AU2001240000A1

Abstract

An optical amplifier (100) with integrated optical waveguide, pump source and other, optional components for amplifying an input optical signal coupled from a fiber optic. The amplifier includes a housing (160) having these components mounted therein, and appropriate optics (120, 130, 150) for coupling the input and output optical signals to and from the appropriate ports, and for carrying an optical pump signal from its source (140) to the waveguide. The optical waveguide disclosed is a channel waveguide amplification chip (110), having a relatively small size; and the pump source disclosed is a laser diode capable of generating the optical pump signal internal to the housing with only electrical (e.g., power) signals applied thereto from outside of the housing. Other optional components may be provided for cooperative optical processing in the amplifier housing such as isolators, gain flattening filters, and optical taps for monitoring. The disclosed optical amplifier offers size and cost advantages over other known systems.

Description

COMPACT OPTICAL AMPLIFIER WITH INTEGRATED

OPTICAL WAVEGUIDE, PUMP SOURCE,

AND PERFORMANCE ENHANCING OPTICAL COMPONENTS

Cross-Reference to Related Applications

This Application relates to the following commonly assigned, previously filed, co-pending U.S. Patent Applications:

1. U.S. Patent Application serial number 09/121,455, filed My 23, 1998 and entitled "Method for Fabricating an Optical Waveguide;"

2. U.S. Patent Application serial number 09/159,012, filed September 23, 1998 and entitled "Optical Channel Waveguide Amplifier;" and

3. U.S. Patent Application serial number 09/316, 102, filed May 24, 1999 and entitled "Compact Optical Amplifier With Integrated Optical Waveguide and Pump Source."

Each of these previously filed U.S. Patent Applications is hereby incorporated by reference herein in its entirety.

Technical Field

The present invention relates to optical amplifiers, and more particularly to a compact, low cost optical amplifier package having a small optical waveguide amplifier chip, optical pump and optimizing optical components integrated therein.

Background of the Invention

The field of optical telecommunications has experienced phenomenal growth over the past several years, fueled in large part by the development and deployment of erbium-doped fiber amplifiers ("EDFAs"). Prior to optical amplifiers, fiber optic communication systems required electronic repeaters to amplify signals and offset the losses associated with long distance optical transmission. These electronic devices converted the transmitted optical signal into the electrical domain, amplified and reshaped the electrical signal, and converted the electrical signal back into an optical signal, and onto the next leg of the fiber system.

Electronic detectors are incapable of discriminating between different wavelengths, therefore, multi- wavelength systems require an independent repeater for each wavelength in the system, as well as the necessary filtering components to isolate each of the wavelengths into their respective repeaters. Fig. la is a representative bock diagram of a 16 channel repeater system 10. The large amount of equipment needed made multi- wavelength communications systems using repeaters prohibitively expensive.

The deployment of the EDFA, however, changed the topology of fiber-optic communication networks. Because the optical amplifier was capable of amplifying multiple wavelengths independently in a single unit, a multi- wavelength system could use a single optical amplifier. The 16 channel repeater system of Fig. la requires a wavelength de-multiplexer 12 to break the 16 channels out onto individual fibers 14, 16 repeaters 18, and a wavelength multiplexer 16 to re- combine the 16 individual wavelengths. All of this was replaced by the single EDFA 20 shown in Fig. lb.

This ability to add multiple channels to a single fiber with minimal additional amplifier costs created an explosion in the bandwidth of optical fiber communication systems. Deployed systems went from single channel sub-gigabit per second operation to 128 channel, multi-gigabit per second per channel operation.

The continuing growth in the bandwidth of optical communication systems has also led to a new demand for more complex functionality in optical networks. To satisfy this demand, manufacturers are desirous of devices in increasingly smaller and more compact packages, while at the same time integrating multiple functions into a single device. In sharp contrast to the ever-shrinking size of many optical network components, the EDFA faces a hard limit because of the fixed value of the minimum bend radius of the fiber. Bend losses are proportional to the bend radius, and hence the radius must be kept large. A nominal radius for such fiber is about 3.75 cm, resulting in a coil diameter of about 7.5 cm (about 3 inches) which in turn results in an minimum package planar dimension of at least 3*/2 x 3¹. inches. Consequently, though many components such as the isolators, 980/1550 nm multiplexers, and monitor taps and photodiodes can be integrated into the package, the footprint remains constrained by the minimum diameter of the erbium-doped fiber coil. Nominal package dimensions are now about 6 6 inches.

Semiconductor optical amplifiers (SOAs) are also available, and rely on electrical (rather than optical) pump sources for amplification. However, their performance characteristics are known to be deficient in many areas, compared to EDFAs.

The demand for more compact devices with greater integrated functionality is accompanied by the need to reduce the cost of these devices. As optical system designers increase the complexity of their networks, the number of optoelectronic components required grows substantially. For the very same reasons that multi- wavelength systems with electronic repeaters were prohibitively expensive, more complex networks begin to face the same cost issues. Thus, not only is "real estate" within the system at a premium requiring smaller, integrated devices, but cost also plays a significant role requiring less expensive components as well.

The EDFA was the critical enabling technology that gave rise to the recent boom in optical communications known as WDM. As communications systems continue to grow and expand, system designers find themselves in need of new devices, amplifiers in particular, that are smaller and less expensive than their current counterparts. A compact, low cost optical amplifier with the ability to incorporate additional devices for added functionality is therefore required.

Summary of the Invention

The shortcomings of the prior approaches are overcome, and additional advantages are provided, by the present invention which in one aspect is an optical amplifier having a housing with optical signal input and output ports; a channel waveguide chip in said housing for optically amplifying an input optical signal from the input port using an optical pump signal applied thereto; and an optical pump source in the housing for generating the optical pump signal.

The amplifier may also include input coupling optics for transmitting the input optical signal into the channel waveguide chip from the input port; and output coupling optics for carrying an output optical signal from the channel waveguide chip to the output port.

The input and/or output optics may include at least one performance enhancing optical component, for example, an optical isolator to minimize resonances, and/or a gain-flattening filter, and/or an optical tap for signal monitoring.

In one embodiment, the channel waveguide chip includes a linear core having an input end for receiving an input optical signal, and an output end for producing an output optical signal; and a surface through which the optical pump signal is received. The surface of the channel waveguide chip may be arranged at an approximately 45 degree angle with the linear core thereof. The optical pump source transmits the optical pump signal at approximately a 90 degree angle with the core. The chip may also include a reflective coating applied over the surface to reflect the optical pump signal into the linear core. A prism may be applied over the surface for directing the input optical signal colinearly into the core. The optical pump source may be a laser diode which generates the optical pump signal internal to the housing, wherein the only optical ports of the amplifier are the optical signal input and output ports.

Based on the types of components used, the present invention attains a package dimension of less than about 3 inches in at least one planar dimension.

The amplifier disclosed herein provides, in a compact, cost-effective package, a channel waveguide amplifier chip, appropriate optics to couple the signal into and out of the waveguide, a pump laser diode, and appropriate optics to focus the light from the pump laser diode into the waveguide. In addition, the amplifier may also be constructed to integrate additional optical components for added functionality. This invention therefore provides significant optical amplification in a low-cost, compact package that can minimize the space required within a communications system to provide signal amplification, and be integrated with additional components, such as splitters, wavelength multiplexers or demultiplexers, or optical add/drop multiplexers.

Brief Description of the Drawings

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following detailed description of the preferred embodiment(s) and the accompanying drawings in which:

Fig. la depicts in schematic form an electrical repeater approach to amplifying a multi-channel optical signal;

Fig. lb depicts in schematic form an EDFA approach to amplifying a multi-channel optical signal; Fig. 2a is a top plan view of an optical amplifier in accordance with the present invention, partially in schematic form and with the cover of the housing removed;

Fig. 2b is a top plan view of another embodiment of an optical amplifier in accordance with the present invention, partially in schematic form and with the cover of the housing removed;

Fig. 3 is a perspective view of a channel waveguide amplification chip employed in the amplifiers of Figs. 2a-b, in accordance with the present invention;

Fig. 4 is a front sectional view of an alternate embodiment of the channel waveguide chip of Fig. 3;

Fig. 5 is another embodiment of an optical amplifier in accordance with the present invention including performance enhancing optical components;

Fig. 6 is a graph of the amplifier gain (without a flattening filter);

Fig. 7 is a graph of the gain vs. input power of the amplifier; and

Fig. 8 is a graph of the output vs. input power of the amplifier.

Detailed Description of the Preferred Embodiment(s)

With reference to Figs. 2a-b, compact waveguide amplifiers 100 and 100' in accordance with the present invention include a channel waveguide amplifier chip 110, signal input coupling optics 120 (to couple the optical input signal from input fiber 122), signal output coupling optics 130 (to couple the optical output signal to output fiber 132), a pump laser 140, and pump coupling optics 150 (to couple the optical pump signal between pump laser 140 and chip 110). Certain components may be aligned on alignment bench 190. Optional components such as planar lightwave circuit 170 and its associated coupling lens 180 can also be easily integrated into amplifier 100 of Fig. 2a, but are omitted from the smaller version thereof 100' of Fig. 2b.

The compact size of the housing 160 (e.g., nickel/gold plated) results from the small waveguide chip 110, coupled with the ability to use an integrated diode pump 140, discussed further below.

Exemplary channel waveguide chip 110, shown in detail in Figs. 3-4, may be of the type disclosed in the above-incorporated U.S. Patent Application 09/159,012 entitled "Optical Channel Waveguide Amplifier," and may be fabricated using the procedures disclosed in the above-incorporated U.S. Patent Application 09/121,455 entitled "Method for Fabricating an Optical Waveguide". The waveguide chip 110 may be composed of a linear core 212 doped with an active ion such as Nd: YAG, Er:glass, or Nd:glass, among others. The compact nature of the waveguide is enabled by the use of high-gain materials. Of particular applicability is the family of Erbium/Ytterbium(Er/Yb) co-doped phosphate glasses which have nearly 100 times the number density of atoms, do not suffer quenching effects, and have twice the emission cross-section relative to their silica counterparts typically used in EDFAs . The core 212 may be nominally square in cross-section and of size fromlO μm to 30 μm on a side. The surrounding cladding 214 may be an undoped material of lower refractive index. One end of the waveguide chip 216 may be polished at an angle (α) of 45 degrees with respect to the waveguide axis. The angle-polished end is then coated 220 to reflect the applied pump signal 211, and overlaid with a prism 230 matched in index to the core (and bonded to the waveguide with an ultraviolet(UV)-cured optical adhesive) for directing the input optical signal colinearly into the core as signal 213. The opposing end of the waveguide may be prepared in one of two ways. The first design (shown in Fig. 4) utilizes a perpendicular end-face 218 with an anti-reflection (AR) coating 222 to minimize back-reflections. If, however, the AR coating is insufficient, the second design option (shown in Figs. 2a-b) involves polishing the opposing end at 45 degrees and bonding an index-matched right- angle prism with UV-cured adhesive to further mitigate reflections.

Channel waveguide chip 110 in one embodiment is about 10-30 mm in length (L), and the overall planar dimensions of the amplifier housing of amplifier 100' (i.e., the length and width of the view of Fig. 2b without the optional PLC 170 and coupling lens 180 of amplifier 100 of Fig. 2a), can therefore be about 1 ^lA inches x about 2 Vz inches. (The package thickness is about 3/4 inch.) This smaller, overall planar dimension stands in contrast to that of a functionally equivalent EDFA amplifier of 6 x 6 inches, at the 1530 - 1565 nm bandwidth of interest, and in fact is even better than the absolute, theoretically planar dimensions of EDFAs discussed above of 3 x 3 inches, based on the minimum bend radius. The present invention therefore provides at least one (and in fact both) planar dimensions of less than about 3 inches. Waveguide chip 110 of amplifiers 100 and 100' provides high gain in a relatively short device (when compared to the several meters of erbium-doped fiber typical in EDFAs), and therefore results in a smaller size of the amplifier housings.

Waveguide 110 requires a pump source to provide a source of the optical pump signal 211 for waveguide gain. With reference to Fig. 2a, exemplary pump source 140 disclosed herein is a single or multi-mode laser diode on a submount and maintained at constant temperature with a thermo-electric cooler (TEC) 142. One exemplary diode is an Open Heat Sink Packaged Laser Diode available from High Power Devices, Inc., model number HPD1005C. Power is provided to both the laser diode and the TEC through external pins 144. Additional pins may also be included to incorporate a monitor photodiode to monitor pump laser power, a thermistor to monitor pump laser temperature, and gain monitoring sensors (not shown). To couple the signal 211 from the pump laser diode into the waveguide 110, lenses 150 are used to focus the power into the waveguide off of the 45-degree coated end-face of the waveguide. The pump source may therefore be arranged to transmit the optical pump signal at an angle of 90 degrees relative the longitudinal axis of the waveguide. Notably, the optical pump signal is generated internal to the amplifier housing, and no additional optical input ports are required for the pump signal. This feature adds to the cost and space savings provided by the amplifier disclosed herein.

Amplifiers 100 and 100' also include the optics used to couple the optical signal into and out of the waveguide 110. The input side of the waveguide is typically buried behind a prism 230 (Fig. 4) and thus requires a set of optics 120 to image the single-mode fiber 122 output into the waveguide chip 110. At a minimum, a low numerical aperture (NA) matched pair of lenses may be used to image the fiber output with a 1 : 1 magnification ratio into the waveguide. Other options include lenses that can expand the beam to fill the waveguide core. In any case, the goal of the input optics is to couple the signal light from the input fiber 122 into the waveguide with as little loss as possible. For core dimensions larger than approximately 10 μm on a side, 1 : 1 imaging lenses work very well.

Coupling optics 130 recover the signal from the waveguide and return it to the optical fiber 132. For waveguides not much larger than 10-15 μm on a side, 1 :1 imaging lenses can effect coupling with as little as 2-3 dB of total coupling loss. To reduce the coupling losses further may require lenses that provide image reduction. Similarly, for larger waveguides some form of imaging reduction may also be necessary.

The compact nature of the amplifier disclosed herein facilitates integration of additional components therein. For example, as shown in Fig. 2a, an optional planar lightwave circuit (PLC) 170 may, for example, be a silica waveguide splitter (e.g., Gould Fiber Optics, model number 47-10335-18-05631) or an arrayed waveguide grating (AWG) multiplexer (e.g., Photonic Integration Research, Inc., model number AWG-WB-lx8-200G-1.5-M-FC). Each of these devices, because of its performance in a filter capacity, has high insertion loss associated therewith. However, when integrated with the compact, low-cost amplifier discussed above in a small package, the intrinsic losses are offset to create so-called "lossless" versions. Many benefits result from a small, inexpensive amplifier integrated with one, or even several, additional optoelectronic components, such as PLC 170.

Because the waveguide 110 (and optional component 170) are coupled to fibers, other fiber coupling devices can be placed in the path to enhance performance. For example, and with reference to Fig. 5, an improved amplifier package 300 is shown therein having improved coupling optics 330. The improved optics (input and/or output) include, for example, a free space optical isolator 332, and/or a thin-film gain-flattening filter 334. The isolator can be used to prevent resonances resulting from unpredictable reflections from cascaded devices (for example, multiple cascaded amplifiers). Also, higher gain devices within the amplifier may produce unwanted reflections, and isolators can also be used to prevent lasing within the amplifier itself.

Also shown in Fig. 5 is a thin-film gain-flattening filter 334. This filter can be obtained with a gain profile inverse to that of the amplifier itself, depicted in Fig. 6, which is the gain profile of the amplifier without a gain-flattening filter employed. The filter results in a flat gain curve. Gain-flattening is desirable, especially between the operating bandwidth of about 1530-1560 nm.

Also depicted in Fig. 5 is an optical tap (e.g., 1% mirror) 336, having an output directed to a photodetector 338. These optional components could be used for monitoring the optical signal as it passes through amplifier 300.

These performance enhancing optical components could be placed in the input optics, output optics, or both, and these terms are used broadly herein to connote any hardware which can be fairly characterized as carrying signals to or from the waveguide. As discussed above, Fig. 6 is a gain profile of the amplifier (without any gain-flattening filters employed). Figs. 7 and 8 are additional plots of the performance of the amplifier (gain versus input power) and (output power versus input power), respectively.

In current implementations, the small signal gain of the amplifier is about

10 dB; the noise figure at 1550 mn, -30 dBm is <5 dB; the 5 dB gain bandwidth (without flattening) spans from about 1525 to 1560 nm; the maximum saturated output power is >13 dBm; and the output power at the 3 dB gain compression (1550 nm) is >10 dBm. Offering more than 10 dB of gain and saturated powers above 13 dBm, this amplifier is suitable for overcoming losses associated with couplers, WDMs, switches and other passive optical components necessary in optical networking systems. Its applications include metropolitan networks; lossless branching; testbeds; instrumentation; optical add-drop multiplexers; and optical switch arrays.

The amplifiers discussed herein, based on a channel waveguide chip, provide compact, low cost optical solutions for use in fiber optic systems. Their primary application is optical amplification in communication systems where space is at a premium, and smaller devices are required. In addition, these amplifiers are ideal for use in systems where the design requires large numbers of low-cost devices to achieve the desired performance. Finally, because of their compact nature, the amplifiers can be integrated with other devices such as splitters or multiplexers and de-multiplexers. When adding additional components into the package, the insertion loss associated with the components can be offset by the optical amplification, allowing for the development of "lossless" versions of these very same optical components.

While the invention has been particularly shown and described with reference to preferred embodiment(s) thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

ClaimsWhdti is claimed is:

1. An optical amplifier, comprising:

a housing having optical signal input and output ports;

a channel waveguide chip in said housing for optically amplifying an input optical signal from said input port using an optical pump signal applied thereto;

an optical pump source in said housing for generating the optical pump signal; and

^■ input or output coupling optics for transmitting the input . optical signal into or out of the channel waveguide chip from the input or output port, the coupling optics including at least one performance enhancing optical component.

. v 2., The optical amplifier of claim 1, wherein the at least one performance enhancing optical component comprises an optical isolator to minimize resonances, and/or a gain-flattening filter, and/or an optical tap for signal monitoring.

3. The optical amplifier of claim 1 , wherein the channel waveguide chip comprises:

a linear core having an input end for receiving an input optical signal, and an output end for producing an output optical signal; and

a surface through which the optical pump signal is received.

4. The optical amplifier of claim 3, wherein the surface of the waveguide amplification chip is arranged at an approximately 45 degree angle with the linear core thereof, the optical pump source transmits the optical pump signal at approximately a 90 degree angle with the core, the channel waveguide chip further comprising:

a reflective coating applied over the surface to reflect the optical pump signal into the linear core.

5. The optical amplifier of claim 4, wherein the channel waveguide chip further comprises:

a prism applied over the surface for directing the input optical signal colinearly into the linear core.

6. The optical amplifier of claim 1, wherein the optical pump source is a laser diode which generates the optical pump signal internal to the housing, and wherein the only optical ports of the amplifier are the optical signal input and output ports.

7. The optical amplifier of claim 1, wherein the housing has at least one planar dimension less than about 3 inches.

8. The optical amplifier of claim 1, further comprising:

an additional optical component in the housing in cooperative operation with the channel waveguide chip.

9. A method for optically amplifying an input optical signal applied to an amplifier housing, the method comprising:

generating an optical pump signal in the housing;

applying the optical pump signal to a channel waveguide chip in the housing to facilitate amplification of the input optical signal into an amplified, output optical signal; and

coupling, using coupling optics, the input or output optical signal from a fiber optic to of from the channel waveguide chip, the coupling optics including at least one performance enhancing optical component.

10. The method of claim 9, wherein the at least one performance enhancing optical component comprises an optical isolator to minimize resonances, and/or a gain flattening filter, and/or an optical tap for signal monitoring.

11. The method of claim 9, wherein said generating includes using a laser diode in said housing.

12. The method of claim 9, further comprising coupling as optical signals to the housing only the input optical signal and the output optical signal.

13. The method of claim 9, wherein the channel waveguide chip comprises:

a surface through which the optical pump signal is received.

14. The method of claim 13 , wherein the surface of the channel waveguide chip is arranged at an approximately 45 degree angle with the linear core thereof, the optical pump source transmits the optical pump signal at approximately a 90 degree angle with the core, the channel waveguide chip further comprising:

15. The method of claim 14, wherein the channel waveguide chip further comprises:

16. A method for fabricating an optical amplifier, comprising:

providing a housing having optical signal input and output ports;

positioning a channel waveguide chip in said housing for optically amplifying an input optical signal from said input port using an optical pump signal applied thereto;

positioning an optical pump source in said housing for generating the optical pump signal; and

providing input or coupling optics for transmitting the input or optical signal into or out of the channel waveguide chip from the input or output port, the optics including at least one performance enhancing optical component.

17. The method of claim 16, wherein the at least one performance enhancing optical component comprises an optical isolator to minimize resonances, and/or a gain flattening filter, and/or an optical tap for signal monitoring.

18. The method of claim 16, wherein the channel waveguide chip comprises:

a surface through which the optical pump signal is received.

19. The method of claim 18, wherein the surface of the waveguide amplification chip is arranged at an approximately 45 degree angle with the linear core thereof, the optical pump source transmits the optical pump signal at approximately a 90 degree angle with the core, the channel waveguide chip further comprising:

20. The method of claim 18, wherein the channel waveguide chip further comprises:

21. The method claim 16, wherein the optical pump source is a laser diode which generates the optical pump signal internal to the housing, and wherein the only optical ports of the amplifier are the optical signal input and output ports.

22. The method of claim 16, wherein the housing has at least one planar dimension less than about 3 inches.

3. The method of claim 16, further comprising:

providing an additional optical component in the housing for cooperative operation with the channel waveguide chip.