光学接收器的灵敏度英文.docx

1、光学接收器的灵敏度英文 Optical-Receiver SensitivityIn addition to the effective delivery of the signal to the detector, the performance of the optical link also depends on the receiver sensitivity (measured in terms of received photons per bit). Because of the high cost associated with increasing the transmit

2、power and system aperture, improving the receiver sensitivity is an important factor in the deep-space laser com system design.Either a coherent receiver or a direct-detection receiver can be used to detect the optical signal. In a coherent optical receiver, the incoming signal is mixed with the out

3、put of a strong local oscillator (LO) beam, and the interference between the signal and LO in the combined field is detected using a pair of photo detectors. Figure2-4 shows a conceptual block diagram of a coherent receiver.The mixing of the weak signal field and the strong LO field at the frontend

4、of a coherent receiver provides linear amplification and down-converts the optical signal into an electrical output at the intermediate frequency (IF) with gain (usually tens of decibels). With a sufficiently strong LO field, this raises the signal level well above the noise level of subsequent elec

5、tronics. The sensitivity of the coherent receiver is thus limited by the self noise (i.e., signal shot noise) of the incident signal. Furthermore, because of the spatial mixing process, the coherent receiver is sensitive only to signal and background noise that falls within the same spatial-temporal

6、 mode of the LO. A coherent receiver can, in principle, operate with a very strong background (e.g., with the Sun in the field of view) without significant performance degradation.Where s is the rate of detected signal photons, and the last approximation was made in the limit of large signal bandwid

7、th B. Equation (2.2-4) states that the limiting capacity of a heterodyne optical channel is -1.44 bits per detected photon.Even though the coherent receiver can in principal provide near-quantum limited receiver sensitivity, such performance is achieved only through near perfect spatial-mode matchin

8、g between the incoming signal and the LO. The added complexity to accomplish the spatial wavefront matching can be very difficult to achieve for a ground-based receiver. This is because the atmosphere effectively breaks up the incident wavefront into a number of coherent cells of sizes approximately

9、 the coherence length of the atmosphere . The size of , under typical operating condition, is on the order of 5-30 cm. Although adaptive optics techniques have been developed to partially compensate for the wavefront distortion, effective wavefront correction over the large apertureLink and System D

10、esigndiameter envisioned for the deep-space receiver will require an active mirror with a large number of actuators. Because of the complexity of such a system, and because the simpler direct-detection receivers have managed to achieve similar, if not better performance, coherent receivers are not b

11、eing considered for a ground-based receiver. Instead, the bulk of the development has been focused on the direct-detection receiver.In a direct-detection receiver, the received optical intensity is detected without extensive front-end optical processing. Figure 2-5 shows a conceptual block diagram o

12、f a direct-detection receiver. The incident signal is collected by the receive telescope. A polarization filter followed by a narrowband filter, and a field stop effectively reduces the amount of background noise incident onto the detector.The capacity of a direct-detection optical link has been stu

13、died extensively.When the receiver is capable of detecting individual photons, Pierce 2 first showed that the capacity of the optical channel can be improved by using a modulation format with very high-bandwidth expansion ratios. Subsequent work by Wyner 3 showed that the capacity of a direct detent

14、ion optical channel in the presence of background can be written as:where s is the rate of arrival for the detected signal photon (measured in photordsec), = s /AB is the (detected) peak signal to background power ratio and M is the peak-to-average power ratio of the signal.Figure 2-6 shows a plot o

15、f the channel capacity versus the peak-to-average signal ratio for several values of the average signal-to-background noise ratios. It is possible to transmit more than 1 bit/photon at a sufficiently high peak-to-average power ratio 12,13. In other words, a photon-counting direct-detection receiver

16、can achieve a higher channel capacity than a coherent receiver by using modulation formats that exhibit high peak-to-average power ratios. shows that the capacity of a direct detection optical link using ideal photon-counting detector can be improved by1 ) Improving s, or equivalently, increasing th

17、e photon detection efficiency for a given receive optical power level,2) Increasing M , the peak to average power ratio: the performance of the direct detection optical channel can be improved by selecting a modulation format that maintains a high peak to average power ratio,3) Improving , the signa

18、l to noise power ratio by limiting the amount of background optical power detected by the photodetector. Even though Eq. (2.2-5) was derived from an ideal photon-counting receiver model, the general behavior of the channel capacity remains valid for a wide range of receiverddetectors that are shot-n

19、oise limited. That is, the performance of the direct-detection link can be improved by increasing the detector sensitivity, selecting a modulation format with high peak to average power ratio, and reducing the amount of background light detected. Each of these factors is briefly described below.1 .P

20、hoton Detection SensitivityImproving the photon detection efficiency is an obvious method of improving the channel performance. For a direct-detection receiver, this is generally accomplished by using detectors with internal amplifications, such as avalanche photodiodes and photomultiplier tubes.In

21、the limit of a very high amplification gain, the receivers noise contribution can be ignored, and the receiver is capable of discriminating the individual photon arrival events and counting photons. If the detector contribute negligible amount of dark counts, such a receiver is capable of achieving

22、the channel capacity shown in. For a more general class of optical receiver that is not capable of discriminating individual photon arrivals, the channel capacity will depend on the noise added by the receiver, including the noise introduced by the amplification process and the thermal noise from th

23、e circuit elements. Even if the receiver is not photon-counting, improving the receiver sensitivity can still result in a corresponding increase in the channel capacity. This is accomplished by increasing the detector amplification while controlling the noise introduced by the amplification process

24、(e.g ., excess noise) and the thermal/leakage current noise. Refer to Section 6.2 for more detailed discussion of the photon detection.2 .Modulation Format.One practical modulation format to achieve high peak-to-average-power ratio is the M-ary pulse-position modulation (PPM). In an M-ary PPM modula

25、tion scheme, each channel symbol period is divided into M timeslots, and the information is conveyed through the channel by the time window in which the signal pulse is present. An illustration of the PPM modulation for a simple case of M = 8 is shown in Fig. When the transmit laser exhibits a suffi

26、cient modulation extinction ratio, the peak-to-average power ratio of an M-ary PPM channel is equal to M, and the capacity of the M-ary PPM channel closely approximates the ideal Poisson channel capacity stated in. Additionally, when M = 2 k , each PPM channel symbol can be mapped directly to a k-bi

27、ts sequence, thus simplifying the bit-to-symbol mapping problem. For these reasons, except when thetransmitter is peak-power limited or when the system is modulation-bandwidth limited, most deep-space optical links analyzed to date had assumed M-ary PPM modulations. .3 .Background Noise ControlThe d

28、iscussion following shows that the performance of the direct detection channel can be improved by reducing the amount of background noise detected by the receiver. For a typical ground based receiver, the sources of background noise include:1) Diffused (extended) background from the atmosphere, The

29、background irradiance from the extended background can be written aswhere L() represents sky radiance, which is a function of wavelength and solar illumination geometry, Ar is the effective receiver area, is the solid angle field of view in steradians, Ar is the optical bandpass, and R is the effici

30、ency of the optical receiving system. 2) Planetary or stellar background objects within the receiver field of view. For a point source (e.g., a star) in the receiver field of view, the amount of background power collected by the receiver is written aswhere H() is the spectral irradiance of the backg

31、round source, with units of watts per meter squared. micron.3) In addition to the point sources and extended background sources, another majorsource of background photons is the scattered light collected by the receive optics. A strong background source near the field of view of the receiver can lea

32、d to significant scattering into the receiver field of view. For an optical receiver design with optics under direct exposure to sunlight, the scattering contribution is one of the major background noise sources The amount of scattered sunlight collected by the receiver can be written aswhere T ( )

33、represents the atmospheric attenuation and I represents the exo-atmospheric solar constant (0.074 W/cm*pm) and BSDF() is the bidirectional scatter distribution function as a function of incident angle. The BSDF values depend on .the surface micro roughness and contamination levels and, in general, they exhibit a pow