【正文】 d additionally, in non directed propagation, it is difficult to achieve the required mixing efficiency. Instead, one can use direct detection using a photodetector. The photodetector current is proportional to the received optical signal intensity, which for intensity modulation, is also the original modulating signal. Hence, most systems use intensity modulation with direct detection (IM/DD)to achieve optical modulation and demodulation. In a freespace optical munication system, the detector is illuminated by sources of light energy other than the source. These can include ambient lighting sources, such as natural sunlight, fluorescent lamp light, and incandescent lamp light. These sources cause variation in the received photocurrent that is unrelated to the transmitted signal, resulting in an additive noise ponent at the receiver. We can write the photocurrent at the receiver as where R is the responsivity of the receiving photodiode (A/W). Note that the electrical impulse response c(t) is simply R times the optical impulse response h(t). Depending on the situation, some authors use (t) and some use h(t) as the impulse response. B. Receivers and Transmitters A transmitter or source converts an electrical signal to an optical signal. The two most appropriate types of device are the lightemitting diode (LED)and semiconductor laser diode (LD).LEDs have a naturally wide transmission pattern, and so are suited to non directed links. Eye safety is much simpler to achieve for an LED than for a laser diode, which usually have very narrow transmit beams. The principal advantages of laser diodes are their high energyconversion efficiency, their high modulation bandwidth, and their relatively narrow spectral width. Although laser diodes offer several advantages over LEDs that could be exploited, most shortrange mercial systems currently use LEDs. A receiver or detector converts optical power into electrical current by detecting the photon flux incident on the detector surface. Silicon pin photodiodes are ideal for wireless infrared munications as they have good quantum efficiency in this band and are inexpensive(4). Avalanche photodiodes are not used here since the dominant noise source is background lightinduced shot noise rather than thermal circuit noise. C. Transmission Wavelength and Noise The most important factor to consider when choosing a transmission wavelength is the availability of effective, lowcost sources and detectors. The availability of LEDs and silicon photodiodes operating in the 800 nm to 1000 nm range is the primary reason for the use of this band. Another important consideration is the spectral distribution of the dominant noise source: background lighting. The noise N(t) can be broken into four ponents: photon noise or shot noise, gain noise, receiver circuit or thermal noise, and periodic noise. Gain noise is only present in avalanchetype devices, so we will not consider it here. Photon noise is the result of the discreteness of photon arrivals. It is due to background light sources, such as sun light, fluorescent lamplight, and incandescent lamp light, as well as the signal dependent source X(t) c(t). Since the background light striking the photo detector is normally much stronger than the signal light, we can neglect the dependency of N(t) on X(t) and consider the photon noise to be additive white Gaussian noise with twosided power spectral density where q is the electron charge, R is the responsivity, and Pn is the optical power of the noise (background light). Receiver noise is due to thermal effects in the receiver circuitry, and is particularly dependent on the type of preamplifier used. With careful circuit design, it can be made insignificant relative to the photon noise(5). Periodic noise is the result of the variation of fluorescent lighting due to the method of driving the lamp using the ballast. This generates an extraneous periodic signal with a fundamental frequency of 44 kHz with significant harmonics to several MHz. Mitigating the effect of periodic noise can be done using highpass filtering in bination with baseline restoration(6), or by careful selection of the modulation type, as discussed in Section . D. Safety There are two safety concerns when dealing with infrared munication systems. Eye safety is a concern