Patent Application
20070258722
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.
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.
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.
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.
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 (>100) and high speed sufficient to meet the requirements of 10 Gbps communications.
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.
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.
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.
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.
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<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.
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.
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.
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.
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.
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.
| Number | Date | Country | |
|---|---|---|---|
| 60798400 | May 2006 | US |
