Abstract:
An optical receiver for enhanced optical power sensitivity for optical signal at 10 Gbps includes an optical package and a supporting electrical circuitry. The optical package includes a semiconductor optical amplifier to pre-amplify the incoming weak signal, a tunable optical filter to suppress the spontaneous noise of the amplifier and a PIN diode as an optical detector. A supporting electrical circuitry includes a control loop for the filter to track the peak of the optical signal. By optimizing the parameters of all the elements, the final sensitivity of the optical receiver can be increased significantly. The device may be realized in a single package.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 60/798,400, filed May 8, 2006, which is incorporated herein by reference in its entirety. 
     
     TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to receivers for fiber optic communications. 
       BACKGROUND 
       [0003]    With the progress of data communication over optical fiber links, both the data rate and transmission distance are increasing. Currently, 10 gigabits per second (Gbps) is becoming more popular for a transmission data rate as an upgrade from systems working at 2.5 Gbps. It may be used in field deployment for backbone networks as well as for local data access networks. Increasing sensitivity for optical receivers at 10 Gbps is always desired. For example, optical receiver detectivity for 2.5 Gbps is about −32 dBm. As many systems at 2.5 Gbps are upgraded to 10 Gbps, one concern is weak sensitivity of the optical receiver at 10 Gbps. The best current optical receiver sensitivity is about −26 dBm, i.e., about 6 dB worse than at 2.5 Gbps. A conventional solution to make the receiver capable of detecting weaker optical signals may be to put an additional erbium-doped fiber amplifier (EDFA) to pre-amplify the signal. An optical detector may then be capable of detecting the amplified signal. However, an EDFA may typically be a large device, i.e., a rack mounted package requiring a 19 inch wide cabinet slot or a “blade” mounted board inserted in a rack assembly. Therefore, there is a need for a compact optical receiver solution to increase the detection capability of the optical signal. 
       SUMMARY 
       [0004]    Systems and methods are disclosed herein to provide a compact optical receiver solution to increase the detection capability of the optical signal. For example, in accordance with an embodiment, a sensitivity enhanced optical receiver includes an optical amplifier, a tunable optical filter, a diode optical detector, and a trans-impedance amplifier. 
         [0005]    In accordance with another embodiment, a optical receiver system includes a sensitivity enhanced optical receiver, a thermoelectric cooler, and a supporting circuit to track the optical peak of the signal and adjust the temperature of the thermoelectric cooler, wherein the central wavelength of the tunable optical filter is temperature tunable and is maintained at the peak of the optical signal by adjusting the temperature. 
         [0006]    The scope of the disclosure is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  shows a block diagram illustrating sensitivity enhanced optical receiver in accordance with an embodiment. 
           [0008]      FIG. 2  shows an embodiment of the sensitivity enhanced optical receiver in a butterfly package. 
           [0009]      FIG. 3  shows a block diagram illustrating an optical receiver system in accordance with an embodiment. 
       
    
    
       [0010]    Embodiments and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures. 
       DETAILED DESCRIPTION 
       [0011]      FIG. 1  is a block diagram of a sensitivity enhanced optical receiver (SEOR)  100  according to one embodiment. An optical signal  105  may be provided via an optical fiber connector (not shown) to the input of an optical amplifier  110 . Various optical amplifiers are known in the art, such as an erbium doped fiber amplifier; however a semiconductor optical amplifier (SOA) may be preferred because the small size may permit implementation with several other miniature optical components in a single package. 
         [0012]    The output of optical amplifier  110  may be input to an optical filter  115 . Optical filter  115  may be implemented as a thin film Fabry-Perot filter to pass a narrow bandwidth of wavelengths, thus reducing any out-of-band optical signal that may be generated by, for example, optical filter  115  or signals on other carrier wavelengths. Filtering the amplified signal in this manner improves the optical signal-to-noise-ratio (OSNR), thus limiting the amount of noise introduced in the system and improving the purity and bit-error rate of the signal. Optical filter  115  can be configured to have the maximum of its bandwidth centered at the optical signal of interest. Since it may occur that many optical wavelength channels are available, it may be desirable for optical filter  115  to be made tunable over a range of wavelengths and may be implemented in various ways. 
         [0013]    One method of tuning optical filter  115 , for example, assuming the filter is a fixed thin film device, depends on the fact that such thin film devices are sensitive to temperature. Therefore, sensitivity enhanced optical receiver  100 , or only optical filter  115  portion of receiver  100 , may be mounted on a thermoelectric heater (described below) that may be controlled to change and control the temperature of optical filter  115  according to a known dependence of peak wavelength transmission vs. wavelength. In this way, sensitivity enhanced optical receiver  100  can be used to track a single wavelength optical signal or switch to another wavelength and track it in the same manner. Alternatively, optical filter  115  may be dynamically tuned and implemented with micro-electromechanical system (MEMS) technology. For any type of Fabry-Perot optical filter, the selectivity is specified by the free spectral range (FSR), which describes the passband bandwidth and separation between successive passbands. The FSR is designed to satisfy the requirements for processing 10 Gbps signals. The FSR may depend, typically, at least on the reflectivity of surfaces or layers in a multi-layer structure, cavity length, mode control, and absorption in the materials through which the light signal passes. 
         [0014]    The output of optical amplifier  110  may optionally first be input to an optical isolator  135 . Optical isolator  135  functions to prevent reflection of the forward transmitted optical signal backwards in an optical system. In this case, a reflection of the amplified signal from optical amplifier  110  back to optical amplifier  110  may cause unstable oscillation in the output of optical amplifier  110 , a common occurrence in such gain systems, which is avoided by introduction of optical isolator  135 . 
         [0015]    The output of optical filter  115  may be the input to a detector  120 . Various detectors are known in the art. For example, detector  120  may be a PIN diode. A PIN diode is a diode with a wide, undoped intrinsic semiconductor region between p-type semiconductor and n-type semiconductor regions. They are not limited in speed by the capacitance between n and p region anymore, but by the time the electrons need to drift across the undoped region. Thus, PIN diodes may be made sufficiently fast to perform at 10 Gbps. Alternatively, avalanche photodiodes (APDs) may be used as detector  120 . APDs are photodetectors that may be reversed biased to provide significant gain (&gt;100) and high speed sufficient to meet the requirements of 10 Gbps communications. 
         [0016]    The output of detector  120  may be a trans-impedance amplifier (TIA)  125 . TIA  125  may provide the gain required and output an electrical signal  130  at an impedance level compatible with electronic signal processing. 
         [0017]    Sensitivity enhanced optical receiver  100  may often deal with optical signals of very low optical power at 10 Gbps. This power level may be well below the sensitivity power of APDs at 10 Gbps, which, for conventional devices, is considered to be about −26 dBm (i.e., 26 dB below 1 mW of optical power). The signal  105  of low optical power may be first fed to semiconductor optical amplifier  110  to boost its power. Semiconductor optical amplifier  110  may be a Fabry-Perot semiconductor laser with anti-reflection coating on both end of the cavity. Because of the absence of high reflectivity end coatings, there is no lasing. In addition, semiconductor optical amplifier  110  may be polarization independent. In order to make the amplification range stable, a thermoelectric heater/cooler (not shown) may be used to hold the amplifier device at a fixed temperature to maintain stable output. 
         [0018]    The output from semiconductor optical amplifier  110  may then be adjusted to be in an acceptable dynamic range of the photo detector. Because of the gain of semiconductor optical amplifier  110 , the output power may be higher than the minimum requirement of PIN detector  120 . Therefore, a PIN device can be used for low cost. An APD may generally be more expensive, which may increase the cost of receiver  100  significantly. 
         [0019]    In order to improve the detected signal-to-noise-ratio, optical filter  115  is used to block the broadband amplified spontaneous emission. The electrical output of photo detector  120  is fed to a trans-impedance amplifier to maximize signal integrity of the output from the detector. 
         [0020]    The following example illustrates how sensitivity enhanced optical receiver  100  can realize power sensitivity. Current commercially available optical APD detectors have power sensitivity superior to PIN diodes, but are generally more costly. APDs may satisfy a minimum power requirement of −26 dBm for a 10 Gbps signal, which is a typical required input optical power level to support a bit error rate (BER) of less than 1 e-12. In order to realize substantially error free transmission (i.e., BER&lt;1 e-15), the optical power level should be at least 2 or 3 dB higher. If the receiving optical signal  105  power is lower than −26 dBm, it may be necessary to first amplify optical signal  105  before outputting it to detector  120 . Another requirement may be to have a sufficient OSNR. 
         [0021]    As an example, assume semiconductor optical amplifier  110  has a gain of 30 dB for a receiving optical signal  105  of −30 dBm. The output power of the signal is 0 dBm, i.e., 1 mW. To achieve minimum OSNR of 20 dB, the noise level at the resolution bandwidth of 0.1 nm should be less than −20 dBm, i.e., less than 0.01 mW. Considering that the noise spreads over a typical amplifier bandwidth range of 50 nm, the integrated noise is 0.01 mW×(50/0.1)=5 mW. Adding a signal power of 1 mW, the total power is 6 mW. This is the requirement of the semiconductor optical amplifier, 30 dB gain and 6 mW saturation power. In this case, however, the input power to the PIN may be greater than the PIN overload limit. Therefore, extra attenuation may be added before outputting optical signal to the PIN diode. 
         [0022]      FIG. 2  is a butterfly package  200  embodiment of the sensitivity enhanced optical receiver in accordance with the disclosure. Receiver butterfly package  200  may differ from conventional butterfly packages for lasers and transmitters in that an output  230  is a differential output  230 - 1  and  230 - 2  to provide high speed is at the output of the optical receiver. Like a standard butterfly package, optical signal  105  may be admitted through a connector  204  that includes a lens (not shown) and an optical isolator (not shown). The lens may be one of various types known in the art, and may include, for example, a Selfoc™ or a ball lens. The isolator typically functions to suppress reflections back to the source or points in the transmission system where reflections might arise, thus causing signal instabilities due to laser feedback or standing waves. A 1 mm ball lens  206 - 1  may be used to couple input optical signal  105  from the fiber holder to semiconductor optical amplifier  210 . Semiconductor optical amplifier  210  may be about 2 mm long. 
         [0023]    At a wavelength of 1550 nm, the gain of semiconductor optical amplifier  210  may be typically about 22 dB. For example, if the input to the amplifier is −32 dBm, output power is then −10 dBm, well above the sensitivity power of a high speed PIN photodiode, which may require a signal greater than −19 dBm to operate. The optical signal may then be coupled to another isolator  235  followed by coupling to a tunable optical filter  215  with ball lens  206 - 2 . Isolator  235  may function to suppress instability inducing reflections back into semiconductor optical amplifier  210 . A typical minimized isolator is about 2 mm long with isolation beyond 30 dB. A micro-electromechanical systems (MEMS) based optical tunable filter can be used as tunable optical filter  215  here to take advantage of small size. A typical MEMS tunable Fabry-Perot (FP) filter is less than 2 mm. The 3 dB bandwidth of the filter may be about 20 GHz. The free spectral range (FSR) of tunable FP filter  215  may be comparable to the range of the broadband noise. With semiconductor optical amplifier  210 , the wavelength bandwidth of the noise is typical 40 to 60 nm. With such parameters, a tunable filter may be achieved. 
         [0024]    The output of tunable optical filter  215  may be coupled to a detector  220 , which may be a PIN diode or an APD, depending on power levels and budget, through ball lens  206 - 3 . A PIN diode detector  220  having a sub-mount of 2 mm length is commercially available. The PIN converts optical signal to electrical current. The output of the PIN is connected to a trans-impedance amplifier (TIA) chip  225 , which converts current to an appropriate voltage level. TIA chips are commercially available for high speed optical photodiode impedance conversion. The length of a typical TIA chip may be about 1 mm. Furthermore, such TIA chips may commonly have differential outputs. They provide the electrical output signal of SEOR  100 . The total length of the elements within butterfly package  200  may be about 14 mm, which is sufficiently less than the inside length of a butterfly package of about 20 mm. 
         [0025]      FIG. 3  shows a block diagram illustrating an optical receiver system  300  in accordance with an embodiment. Referring to  FIGS. 1 and 2 , optical signal  105  enters sensitivity enhanced optical receiver (SEOR)  100 , where it is optically amplified, filtered, detected and trans-impedance amplified. A portion of output electrical signal  130  is monitored by a controller  320  that adjusts the power to, and therefore the temperature of, a thermoelectric heater/cooler  310 . The temperature control provided by thermoelectric heater/cooler  310  adjusts the center of the passband of optical filter  115  to track the wavelength of optical signal  105  to maintain maximum signal. Other means of tuning the passband of optical filter  115  may alternatively be implemented. For example, a MEMS FP may be driven by controller  320 . Additionally, if signal saturation conditions are exceeded, controller  320  may be adapted to provide attenuation to prevent overload of diode detector  120  by intentionally detuning optical filter. 
         [0026]    Embodiments described above illustrate but do not limit the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims.