Free-space optical module for optical amplification

An integrated optical-amplification module includes a housing member, a first input optical terminal configured to receive an optical signal, a second input, optical terminal that can receive a pump light, and an output optical terminal that can output a combined optical signal comprising at least a portion of the optical signal and a portion of the pump light. The integrated optical-amplification module also includes an optical combiner fixedly installed relative to the housing member. The optical combiner can receive the pump light and the optical signal and an optical prism fixedly installed relative to the housing member. The optical combiner can merge the pump light and the optical signal to form the combined optical signal. The optical prism can direct at least a portion of the optical signal through free space to the optical combiner.

BACKGROUND

The present disclosure relates to optical-amplification modules for high-power optical applications.

The invention of optical fiber amplifier is a significant milestone in fiber communication history. Before the appearance of fiber amplifier, the repeaters in fiber communication system had been implemented using opto-electrical and electro-optical transmitters. The optical communication systems were complicated, inefficient, expensive, and sometimes unpredictable. Subsequent research efforts have been devoted to all-optic repeaters such as Raman amplifier, semiconductor amplifier, and Rare-earth doped fiber amplifier. Fiber amplifiers have demonstrated superior performance including high gain, high saturation output power, and low noise levels. These advantages have has made fiber amplifiers key components in fiber communication systems.

Conventional optical fiber amplifiers can be implemented in different pumping configurations. Referring toFIG. 1A, a fiber laser system100includes an input optical fiber11, an isolator12, an optical coupler13A, a fiber amplifier15, another isolator16, a gain flatting filter device17, and an output optical fiber18. The isolator12, the optical coupler13A, the fiber amplifier15, the isolator16, and the gain flatting filter device17can be sequentially coupled by optical fibers. The fiber amplifier15can be implemented by an erbium doped fiber. The optical coupler13A can be implemented by a wavelength division multiplexer (WDM). The input optical fiber11is configured to receive a signal laser beam at a wavelength λ1. The optical coupler13A is coupled to a fiber14that is configured to receive a pump laser beam at a wavelength λ2. The fiber amplifier15can amplify the signal laser beam using the energy provided by the pump laser beam received from the upstream direction. Since the pump laser beam is coupled into the fiber amplifier15from the upstream direction relative to the fiber amplifier15, the optical coupler13A can be said to be in a forward pumping mode. The gain flatting filter device17is used to flatten or smoothen out signal intensities over a specified wavelength range and to ensure uniform gains in different wavelength channel.

In another laser pumping configuration, referring toFIG. 1B, the fiber laser system110includes an optical coupler13B positioned downstream relative to the fiber amplifier15. The fiber amplifier15can amplify the signal laser beam using the energy provided by the pump laser beam from the downstream direction. The fiber laser system110can be said to have a backward pumping mode.

In another laser pumping configuration, referring toFIG. 1C, the fiber laser system120includes an optical coupler13A positioned upstream relative to the fiber amplifier15and an optical coupler13B positioned downstream relative to the fiber amplifier15. Pump laser beams wavelength λ2can be respectively coupled in to the optical coupler13A and the optical coupler13B via the optical fibers14A and14B. The fiber amplifier15can amplify the signal laser beam using the energy provided by the pump laser beam received from the optical fiber14A in the upstream direction, and by the energy provided by the pump laser beam from the optical fiber14B in the downstream direction. The fiber laser system120thus has a bidirectional pumping mode which includes both a forward laser pump and a backward laser pump.

Different optical components in the above described fiber laser systems100,110,120are typically connected by fiber splicing. Drawbacks of these conventional laser systems include: complex configuration, low reliability due to too many splicing joints, inflexibility, and high cost. These conventional laser systems also suffer large insertion losses. The insertion loss for each fiber joint can be in the range of 0.1-0.2 dB. The described conventional laser systems each have 10 or more fiber joints, which can generate more than 1 dB optical power loss. As fiber network is becoming increasingly more complex, reliability and cost become more important. There is a need for a laser system and/or components for laser systems which are highly reliable, and low cost, and easy to be integrated into an optical device.

SUMMARY

In a general aspect, the present invention relates to an integrated optical-amplification module that includes a housing member; a first input optical terminal that can receive an optical signal; a second input optical terminal that can receive a pump light; an output optical terminal that can output a combined optical signal comprising at least a portion of the optical signal and a portion of the pump light; an optical combiner fixedly installed relative to the housing member, wherein the optical combiner can receive the pump light and the optical signal, wherein the optical combiner can merge the pump light and the optical signal to form the combined optical signal; and an optical prism fixedly installed relative to the housing member and being separated from the optical combiner and the first input optical terminal by free space, wherein the optical prism can receive the optical signal and direct at least a portion of the optical signal to the optical combiner.

In another general aspect, the present invention relates to an integrated optical-amplification module that includes a housing member; a first input optical terminal that can receive a first optical signal; a second input optical terminal that can receive a first pump light; a first output optical terminal that can output a first combined optical signal comprising at least a portion of the first optical signal and a portion of the first pump light; a first optical combiner fixedly installed relative to the housing member, wherein the first optical combiner can receive the pump light and the optical signal wherein the first optical combiner can merge the pump light and the optical signal to form the combined optical signal; a first prism fixedly installed relative to the housing member and being separated from the first optical combiner and the first input optical terminal by free space, wherein the first optical prism can receive the first optical signal and direct at least a portion of the first optical signal to the first optical combiner; a third input optical terminal that can receive a second optical signal; a fourth input optical terminal that can receive a second pump light; a second output optical terminal that can output a second combined optical signal comprising at least a portion of the second optical signal and a portion of the second pump light; a second optical combiner fixedly installed relative to the housing member, wherein the second optical combiner can receive the second pump light and the second optical signal, wherein the second optical combiner can merge the second pump light and the second optical signal to form the second combined optical signal; and a second prism fixedly installed relative to the housing member and being separated from the second optical combiner and the third input optical terminal by free space, wherein the second optical prism can receive the second optical signal and direct at least a portion of the second optical signal to the second optical combiner.

In another general aspect, the present invention relates to an integrated optical-amplification module that includes a housing member; a first optical prism fixedly installed relative to the housing member, wherein the first optical prism can include a first surface configured to receive an optical signal from free space and to allow the optical signal to refract through the first optical prism; and a second surface that can receive a pump light from free space and to merge at least a portion of the optical signal refracted through the first optical prism and at least a portion of the pump light to output an merged light in free space; and an amplifier that can amplify the portion of the optical signal in the merged light in response to the portion of the pump light in the merged light to produce an amplified light.

Implementations of the system may include one or more of the following. The optical signal can travel in free space at least a portion of the distance between the first input optical terminal and the optical prism. The first input optical terminal, the second input optical terminal, and the output optical terminal can be fixed to a same side of the housing member. The integrated optical-amplification module can further include an optical isolator positioned between the first input optical terminal and the optical prism, wherein the optical isolator can substantially uni-directionally pass the optical signal from the first input optical terminal to the optical prism. A surface of the optical prism can be coated with a polarization-insensitive thin film. The integrated optical-amplification module can further include a dual core optical collimator that can direct the pump light from the second input optical terminal to the optical combiner and to direct the combined optical signal from the optical combiner to the output optical terminal. The integrated optical-amplification module can further include a photo detector coupled to the optical prism, wherein the photo detector can capture at least a portion of the optical signal and produce an electric signal in response to the optical signal. Embodiments may include one or more of the following advantages. The disclosed systems and methods provide a compact, integrated, and lower cost optical module for optical-amplification applications. Compared to conventional systems, the disclosed optical-amplification module can have much lower insertion loss, higher reliability, is easier for system integration and modularization, and more convenient for system management and optimization. The disclosed optical-amplification includes an assembly of optical components separated by free space and can meet a wide range of applications. Another advantageous feature of the disclosed systems and methods is that the optical-amplification module can provide loading multiplexing, and flexible management of the optical signal and the pump light.

An advantage of the described integrated optical-amplification module is that it can be used in combination with different types of amplifiers. For example, fiber amplifiers having different doping elements and different lengths can be used to receive the combined optical signal. The optical amplification can be conducted at different wavelengths. Another advantage of the described integrated optical-amplification module is that it is compact and easy to use. The optical prism folds the optical paths to allow the input and output optical terminals to be positioned on the same side of the housing member, which reduces foot print and allow easy handling. Another advantage of the described integrated optical-amplification module is that it is flexible. Multiple of optical-amplification modules can be configured in parallel for separate optical amplifications or connected in series for a common optical amplification. Yet another advantage of the described integrated optical-amplification module is that multiple optical components can be packaged and tested in factory, which eliminates time and cost for assembling and testing during device integration.

Although the invention has been particularly shown and described with reference to multiple embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention.

DETAILED DESCRIPTION

Referring toFIG. 2, an optical-amplification module200includes a collimator211configured to receive an optical signal21at wavelength λ1and to produce a collimated optical signal21A. The collimated optical signal21A passes through an isolator22to form optical signal21B that impinges on an optical multiplexer24. The isolator22can block undesirable backward lights and can minimize the interference between optical components, for example, amplified spontaneous emission (ASE) from an amplifier27(as described below) implemented by an erbium doped optical fiber. The optical multiplexer24can be implemented by an optical prism coated with films to make it insensitive to polarizations of the incident light, which can thus reduce polarization dependent loss (PDL). For example, surfaces S1and S2of the optical multiplexer24can be coated with polarization-insensitive films to reduce optical power loss related to directions of the polarizations. The polarization-insensitive films can also be configured to produce desirable splitting ratio between the intensities of the optical signal and pump light. A portion of the optical signal21B is reflected by surface S1of the optical multiplexer24to produce a reflected light241. The reflected light241can be received by a photo detector23that is configured to output an electric signal in response to intensity of the reflected light241. The photo detector23is used monitor the power of the optical signal. Another portion of the optical signal21B transmits through the optical multiplexer24to form a refraction light242which transmits surface S2of the optical multiplexer24to form a part of the light243. The intensities of the reflected light241and the refraction light242can be set by a predetermined splitting ratio.

A pump light25at wavelength λ2is collimated by a collimator212to form a collimated pump light25A that impinges on a optical multiplexer26. The pump light25can for example be a pump laser beam produced by a pump laser. Similar to the optical multiplexer24, the optical multiplexer26can be an optical prism coated with interference films to reduce polarization dependent optical losses. A portion of the collimated pump light25A transmits through the optical multiplexer26as a refracted light262which is not used in the amplification of the optical signal21and is desirably minimized by proper design of the coating on the surface S3. Another portion of the collimated pump light25A is reflected by surface S3of the optical multiplexer26to form a reflected pump light261. The reflected pump light261is reflected by surface S2of the optical multiplexer24to form a portion of the light243. The light243thus comprises the optical signal21at wavelength λ1and the pump light25at wavelength λ2. In other words, the light243is formed by multiplexing the optical signal and the pump light. The light243is coupled into an amplifier27by a collimator213. The amplifier27can be implemented by an erbium doped optical fiber capable of generating amplified spontaneous emission. The amplifier27amplifies the light243to form a light28which passes through a gain flattening filter (GFF)29to form an output light30. The amplifier27can be implemented by for example an erbium doped optical fiber. The gain flattening filter29can reduce gain variations across spectral bands at wavelengths.

Two or more of the collimators211-213, the isolator22, the optical multiplexers24,26, the photo detector23, and the amplifier27can be separated by free space. As described below, one or more of the collimators211-213, the isolator22, the optical multiplexers24,26, the photo detector23, and the amplifier27can be fixedly assembled in a housing member to form an integrated optical module. In the present invention, the term “free space” refers to a gap filled by air or other uniform medium (such as a gas or a liquid). The optical signal and the pump light can transmit between two or more of the above optical components in free space without the use of optical fibers. Comparing to conventional systems, the elimination of optical fibers for light transmissions between these components can significantly reduce insertion loss related to the coupling or splicing of optical fibers and improve performance. In some embodiments, the insertion loss of more than 1 dB in the conventional laser systems can be prevented. The optical-amplification module can also be miniaturized and reduce cost.

The described optical-amplification module can be assembled in an integrated optical module to reduce footprint, lower cost, and to decrease the time and cost for system integration and testing. Referring toFIG. 3, an integrated optical-amplification module300includes optical-amplification modules370,380,390installed in a rigid housing301and on a base308fixed to the rigid housing301. The rigid housing301can for example be made of a metallic or a plastic material. The base308can be made of a ceramic material. The optical-amplification modules370,380,390can be used in parallel for separate amplifications of different or the same input optical signals. The optical-amplification modules370,380,390respectively include optical collimators311,313, and315for receiving optical signals. The optical collimators311,313, and315are therefore input optical terminals. The optical-amplification modules370,380,390also respectively include optical collimators312A,314A, and316A for receiving pump light. The optical-amplification modules370,380,390also respectively include optical collimators312B,314B, and316B for outputting combined optical signals each including a mixture of the pump light and the optical signal. The optical collimators311,313,315,312A,314A, and316A are input optical terminals. The optical collimators312B,314B, and316B are output optical terminals.

The optical-amplification module370includes a single fiber collimator302, an optical isolator304, an optical prism307, a photo detector306, an optical combiner305, and a dual fiber collimator303, which can be fixed to the base308by screws or adhesives such as epoxy. The optical prism307can be an optical prism coated with films to reduce its dependence on the polarizations of the incident light. The optical collimator311can be implemented by an optical fiber that goes through the rigid housing301to couple to the single fiber collimator302. The optical combiner305can be implemented by a WDM filter and in some applications, can be a gain flatten filter (GFF).

In operation, an optical signal is coupled into the optical collimator311and then into the single fiber collimator302. The optical signal passes through the optical isolator304and is directed to the optical prism307. The optical signal is reflected by total reflection by the optical prism307and to be coupled first into the optical combiner305. A portion of the optical signal can be coupled into the photo detector306for monitoring purpose. The photo detector306can produce an electric signal in response to the optical signal.

The optical collimator312A receives a pump light and couples it into the dual fiber collimator303. The optical combiner305receives the pump light from the dual fiber collimator303and the optical signal from the optical prism307(in opposite directions, thus backward pump light coupling). The optical combiner305combines the pump light and the optical signal to output a combined optical signal first through the dual fiber collimator303and then out of the optical collimator312B. The combined optical signal is input to an external amplifier to generate an amplified light.

The optical-amplification modules370,380,390can also be used in series, which in combination is used for a single optical amplification. The combined optical signal output from the optical collimator312B in the optical-amplification module390can be fed into the optical collimator313in the optical-amplification module380. Similarly, the combined optical signal output from the optical collimator314B in the optical-amplification module380can be fed into the optical collimator315in the optical-amplification module370.

An advantage of the described integrated optical-amplification module is that it can be used in combination with different types of amplifiers. For example, fiber amplifiers having different doping elements and different lengths can be used to receive the combined optical signal. The optical amplification can be conducted at different wavelengths. Another advantage of the described integrated optical-amplification module is that it is compact and easy to use. The optical prism folds the optical paths to allow the input and output optical terminals to be positioned on the same side of the housing member, which reduces foot print and allow easy handling. Another advantage of the described integrated optical-amplification module is that it is flexible. Multiple of optical-amplification modules can be configured in parallel for separate optical amplifications or connected in series for a common optical amplification. Yet another advantage of the described integrated optical-amplification module is that multiple optical components can be packaged and tested in factory, which eliminates time and cost for assembling and testing during device integration.

It should be understood that the described integrated optical-amplification modules can be implemented in other configurations without deviating from the spirit of the present invention. An integrated optical-amplification module can include one, two, or more optical-amplification modules that each can perform above described functions. One or more amplifiers can be included in the described integrated optical-amplification module. The layout and the materials of the various optical components can differ from the examples described above.