Another way to reduce turn-on delay is to bias the low-level injection current JB very close to the threshold. But the photon density increase tends to reduce carrier density through stimulated recombination. We have. The phase modulation, generally uses a smaller bandwidth than FM. The adiabatic chirp is caused by the change of phase condition of laser cavity caused by the refractive index change of semiconductor material inside the laser cavity. In order to generate white light at high-current densities with high power efficiency, the blue-photon source can be a LD instead of a LED. As such, the information regarding sidebands, bandwidth and the like also holds true for phase modulation as it does for frequency modulation, bearing in mind their relationship. A more precise analysis has to solve coupled rate Eqs. This can be expanded into a Bessel series as: In this optical output, the fundamental frequency component is 2Ω, which doubles the input RF frequency. We will discuss some other crucial differences between amplitude modulation and frequency modulation like frequency range, quality of transmission etc. The data streams are separated using spatial multiplexing as explained in Chapter 2. The modulation of any carrier in any way produces sidebands. Phase modulation is defined as the process in which the instantaneous phase of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal. To analyze this process, we have to start from the rate equation at the low injection level JB below threshold, where photon density is very small and the stimulated recombination term is negligible in Eq. The demand for wireless communication links of several tens of Gb/s will increase rapidly in the near future. The electronic equalization technique can also be utilized at the transmitter’s side instead of the receiver’s side [211]. Electro-optic modulator power transfer function and field transfer function. 3.3.5 illustrates the operating principle of direct intensity modulation of a semiconductor laser. However, it usually becomes smaller than this maximum value due to the electrical parasitics associated with the packaging. Today’s wireless communication systems typically operate at frequencies below 10 GHz, resulting in limited modulation bandwidth. However, the remaining coherence of the white laser causes the appearance of speckles in the illumination pattern [49]. Frequency Modulation Phase and Frequency Modulation are the two other types of modulation that can be used to impose data onto a carrier frequency. Therefore, the received signal RF spectrum in the electrical domain will have a discrete frequency at 2Ω, which is twice the RF modulating frequency. The second common method to create a white laser beam is to mix the RGB colors of three separate emitting chips by using a diffuser. According to the etendue law, the area of the source multiplied by the solid angle subtended by the entrance pupil of the system as seen by the source either remains constant or increases in a passive optical system. It is important to notice the difference between the power transfer function and the optical field transfer function of an external modulator, as shown in Figure 1.5.6. BPSK We will begin our discussion of digital phase modulation with a review of the fun-damentals of binary phase shift keying It relates to the variations in the phase of the carrier signal: where The nonenhancement of differential gain in unstrained GaAs SQW lasers explains why modulation bandwidth enhancement was not observed in the SQW lasers in comparison with the bulk DH lasers. The additional differential gain enhancement in the MQW lasers explains the observed differential gain enhancement in MQW structures (Uomi et al., 1985, 1987; Takahashi et al., 1991) and the improved high-speed modulation performance in MQW lasers over their SQW and DH counterparts. This modulation technique can also be used to generate quadruple frequency by increasing the amplitude of the modulating RF signal such that πVm/Vπ≈5. For the digital version, see, Learn how and when to remove this template message, https://en.wikipedia.org/w/index.php?title=Phase_modulation&oldid=972754028, Articles lacking sources from December 2009, Creative Commons Attribution-ShareAlike License, This page was last edited on 13 August 2020, at 17:06. However, the peak efficiency of the blue LD is achieved for an electrical power density of about 25 kW/cm2 that is at least three orders of magnitude higher than that of the blue LED. Eq. Applying Eq. (9.6) and (9.8). The phase of a carrier signal is modulated to follow the changing signal level (amplitude) of the message signal. Rongqing Hui, in Introduction to Fiber-Optic Communications, 2020. This optical frequency modulation is linearly proportional to the chirp parameter αlw as. Spatial hole burning is undesirable for a number of reasons, the main one being that it limits the maximum useful output power from a DFB laser, thus limiting the maximum photon density in the laser cavity and the resonance frequency. Schematic of a dual-drive Ti: LiNbO3 Y-branch Mach-Zehnder modulator. In this type of modulation, the amplitude and frequency of the carrier signal remains unaltered after PM. Calculated power penalties by using the frequency response curve of the butterfly-packaged RSOA shown in Figure 23.18 for the cases (a) without equalizer, (b) with ideal FFE, and (c) with ideal DFE. It has been reported that, to maximize the system’s performance, it is necessary to utilize both the pre-emphasis technique at the transmitter and the electronic equalization technique at the receiver [211]. This bandwidth is in principle determined by the carrier lifetime [204]. The challenge here is to receive the required CSI from each of the client receivers. 9.11. At gigahertz frequencies, the frequency-dependent terms will dominate and thus Z=L/C=1/(cCCo) and Nm=cLC=C/Co, where Co is the capacitance per unit length of the structure with all the dielectrics replaced by vacuum. CPW, Coplanar Waveguide. Bin Zhao, Amnon Yariv, in Semiconductor Lasers I, 1999. (3.3.34) can be integrated to find the time required for the carrier density to increase from NB to Nth: Fig. P(t) = Accos[Wct + kpm(t)] Here ‘Ac’ represents the amplitude of the carrier signal ‘Wc’ represents the carrier signal’s angular frequency = 2∏fc And ‘m(t)’ represents the modulating signal 11.6, the optical setup of a laser transmitter that provides white illumination and data speeds in the order of Gb/s is given [3]. Accordingly, there have been some efforts to utilize the MLSE equalizer for increasing the operating speed of the bandwidth-limited RSOA [86,208,209]. Bandwidth Enhancement by Optical Amplitude and Phase Modulation of Injection-Locked Semiconductor Lasers Erwin K. Lau, Hyuk-Kee Sung, Xiaoxue Zhao, Devang Parekh, Young-Kai Chen*, Connie J. Chang-Hasnain and Ming C. Wu Department of Electrical However, if this optical signal is detected by a photodiode, only optical intensity is received while the phase information is removed. The most straightforward type of PSK is called binary phase shift keying (BPSK), where “binary” refers to the use of two phase offsets (one for logic high, one for logic low).We can intuitively recognize that the system will be more robust if there is greater separation between these two phases—of course it would be difficult for a receiver to distinguish between a symbol with a phase offset of 90° and a symbol with a phase offset of 91°. (3.3.12) can be linearized for the small-signal response as, Here for simplicity, we have assumed that the impact of photon density modulation is negligible. Thus, additional optical components might be required for the transformation of the elliptical beam to a circular or uniform beam. Let t n denote the instant of time at which the PM wave experiences a zero crossing( ) In this case the static approximation is valid and the permittivity may be taken as the geometric mean of the permittivities for the relevant axes.). Frequency doubling is relatively easy to explain, as illustrated in Figure 1.5.5. In practical applications, this time delay td may limit the speed of digital optical modulation in optical systems. 180 degree phase shifts as the data shifts state. A CW 3-dB modulation bandwidth of 15 GHz was reported previously in a GaAs/AlGaAs MQW laser (Ralston et al., 1991). PM is used for signal and waveform generation in digital synthesizers, such as the Yamaha DX7, to implement FM synthesis. The cross section of a typical traveling-wave lithium niobate electrooptic phase modulator, which is also the heart of other high-speed devices such as amplitude and polarization modulators, is shown in Fig. Whereas the long interaction length permits the drive voltage to be kept low, the traveling-wave interaction achieves a higher bandwidth than either the total capacitance or optical transit time would otherwise allow. In the case of optoelectronic effects, the intrinsic response time is exceedingly fast (∼10−13 s); therefore, one or more of the other considerations will dominate. (3.15) one of the parameters required for wide modulation bandwidth is a large photon density in the laser cavity. Electro-optic modulator transfer function and input (electrical)/output (optical) waveforms. Injection current is turned on at t = 0 from JB to J2, but both photon density and carrier density will require a certain time delay to build up toward their final values. Phase Modulation: Phase-sensitivity factor k p =π/2 radians/volt. Illustration of the laser turn-delay. Various models and methods exist for computing the characteristics of the transmission line, namely Nm, αA, and Z, and for calculating the overlap of the electrical and optical fields in order to compute Vπ. For example, Figure 23.18 shows the photographs of the RSOAs in TO-can and butterfly packages and their measured frequency responses. In directly modulated LDs, when the injection current is suddenly switched on from below to above the threshold, there is a time delay between the signal electrical pulse and the generated optical pulse. In these DFB lasers, the photon density within the cavity is nonuniform and is much higher at the center of the cavity, where the phase shift region is located. 3.3.6. Here, we have neglected the optical power level at threshold. Figure 23.18. The ratio between the phase modulation and the intensity modulation is referred to as the modulation chirp. The mentioned design techniques were applied in the design of three fully integrated CMOS circuits: a 100-GHz transformer coupled differential amplifier in 90-nm standard CMOS, a 94-GHz differential power amplifier in 45-nm LP CMOS, and a 120-GHz 10-Gb/s phase-modulating transmitter in 65-nm LP CMOS. However, the eye opening can be recovered by applying the electronic equalization technique to the received signal. For semiconductor materials, the refractive index is usually a function of the carrier density, and thus the emission wavelength of a semiconductor laser often depends on the injection current. As with other modulation indices, this quantity indicates by how much the modulated variable varies around its unmodulated level. Laser sources offer significant advantages over LEDs: they do not experience any power efficiency droop but increase at higher injection currents; they provide a larger modulation bandwidth, that is in the order of hundreds of MHz or even of a few GHz [51]; they are more directive and therefore, their beam can be steered more efficiently by using optics; and they have a linewidth6 of the order of 2 nm that is 25 times smaller than that of LEDs, [52] and thus they allow for an optimal use of the optical spectrum with the application of dense WDM [3]. In a practical LD based on InGaAs, a 1-mA change in the injection current would introduce an optical frequency change on the order of 1 GHz, or 8 pm in 1550 nm wavelength window. Fig. Gain optimization can be accomplished by applying a conjugate match to the transistors. Although the optical intensity waveform is frequency-doubled compared to the driving RF signal, the waveform of the output optical field is quasi-linearly related to the input RF waveform. Low loss slow-wave transmission lines and integrated transformers provide a solution for this matching problem. Figure 23.19. Under these conditions there is no backward-propagating electrical wave, and the voltage experienced by a photon as it traverses the modulator may be written as (Korotky and Alferness 1987), where Ω is the angular drive frequency, V0 is the peak-to-peak drive voltage, and η = β e − (Ω /ω0)β0 is a function of the electrical and optical propagation constants, ß e and ß a. The problem with phase shift keying is that the receiver cannot know the exact phase of the transmitted signal to determi… It relates to the variations in the phase of the carrier signal: 11.22 shows an illustrative example. Detailed analysis of laser modulation can be found in (Agrawal and Dutta, 1986). Rongqing Hui, Maurice O'Sullivan, in Fiber Optic Measurement Techniques, 2009. In amplitude modulation, the amplitude of the carrier varies. But in practice, a large-amplitude RF signal at high frequency is usually difficult to generate. It needs a certain period of time for the carrier density to build up and to reach the threshold level. The result is. θ This bandwidth is in principle determined by the carrier lifetime [204] . However, further increasing the modulation bandwidth to 40 GHz appears to be quite challenging, mainly limited by the carrier lifetime as well as the parasitic effect of the electrode. Recently, a 3-dB modulation bandwidth of 21 GHz was demonstrated in a GaAs/AlGaAs MQW laser (Dong et al., 1996). A shallow well increases the free-carrier absorption and carrier temperature (CH) and reduces optical gain. The bandwidth requirement in case of amplitude modulation is very less as compared to frequency modulation. For example, by using an 8-state MLSE receiver, it has been demonstrated that the RSOA having a modulation bandwidth of only 2.5 GHz can be used to generate the 10-Gb/s upstream signal in a bidirectional WDM PON with the maximum reach of 18 km [209]. Phase modulation (PM) is a modulation pattern for conditioning communication signals for transmission.It encodes a message signal as variations in the instantaneous phase of a carrier wave.Phase modulation is one of the two principal forms of angle modulation, together with frequency modulation. The situation for frequency modulated signals is different.The FM sidebands are dependent on both the level of deviation and the frequency of the modulation. PM is very similar to FM, but in PM the phase of the modulated waveform is varied by the instantaneous voltage of the modulating waveform. Also among the more important characteristics of external modulators for nearly all applications is the modulation bandwidth. In this expression for M(v), dependences upon the fundamental characteristics of the electrode transmission line — i.e., the active length L, the microwave index Nm, and the attenuation coefficient α A — are now evident. On the other hand, when this RSOA is mounted in a butterfly package, there is almost no reduction in the modulation bandwidth, as shown in Figure 23.18c. CPM is a bandwidth and power efficient class of modulation schemes particularly attractive for use with nonlinear power amplifiers. Fig. However, this may result in poor extinction ratio of the output optical pulse. Frequency response of direct modulation mainly depends on the carrier dynamics of the LD and > 20 GHz modulation bandwidth has been demonstrated. Δf = 75 KHz fm = 500 Hz BWFM = 2 [75 + (500/1000)] KHz = 151.0 KHz Δf = 75 KHz fm = 5000 Hz BWFM = 2 [75 + (5000/1000)] KHz = 160.0 KHz If the input RF signal is a sinusoid, V(t)=Vmcos(Ωt), then the output is. Low valence offset increases carrier uniformity and thermionic emission transport; however, too low valence offset leads to a shallow well. The inevitable trade-off between the modulation bandwidth and the power efficiency of LEDs restricts the design of both energy-efficient and high-speed transmitters for VLC [3,49]. The electrical propagation constant may be written as ß e = Nmkm + i α A/2, where km is the microwave wavenumber, Nm represents the real part of the microwave index, and αA is the microwave power attenuation coefficient. The FM band is divided between a variety of purposes. The modulation bandwidth of the RSOA is typically in the range of 1–3 GHz. Copyright © 2020 Elsevier B.V. or its licensors or contributors. Referring to the power transfer function shown in Equation 1.5.7, the bias should make the initial phase ϕ0=mπ±π/2. Radhakrishnan Nagarajan, John E. Bowers, in Semiconductor Lasers I, 1999, From the expression for the resonance frequency derived in Eq. The nonenhancement of differential gain in unstrained GaAs SQW lasers explains why, Reference Module in Materials Science and Materials Engineering. Phase Modulation: ( ) ( ), Frequency Modulation: ( ) ( ) t GAM C p f t A t m h t d h t k t h t k u t M Z D D D G f ªº «» ¬¼ ³ We shall focus more on Frequency Modulation in this course and less on Phase Modulation. On the other hand, for optical frequency modulation-based systems such as frequency-shift key (FSK), lasers with large chirp will be beneficial for an increased optical frequency modulation efficiency. Both on-off modulation and linear modulation can be performed. In this method of mod lation theIn this method of modulation, the amplit de of the carrier a e isamplitude of the carrier wave is maintained constant. The optical power of a LD is a function of the injection current, and thus a convenient way to convert an electrical signal into an optical signal is through the direct modulation of the injection current. The result shows that, when no electronic equalization technique is used, this RSOA can be operated at the speed of up to ∼5 Gb/s. (9.9). αlw is the chirp parameter of the laser, which is identical to a well-known linewidth enhancement factor first introduced by Henry (1982), as the spectral linewidth of the LD is also related to αlw which will be discussed in the following section. It is a simple matter to reduce the bandwidth of a scheme if large energy is In other words, QAM transmits information by changing both the amplitude and phase of a carrier wave, thereby doubling the effective bandwidth. The performance achievable by using the electronic equalization technique can be estimated by calculating the penalty induced by the equalizer (PIE), which represents the power penalty when the ideal equalizer (with infinite tap length) is applied. Note: h(t) is the unit impulse response Thus, by using this electronic equalization technique, it is possible to operate the RSOA at >10 Gb/s despite its narrow modulation bandwidth. The peak amplitude and the frequency of the carrier signal are maintained constant, but as the amplitude of the message signal changes, the phase of the carrier changes correspondingly. Therefore, phase modulation facilitates highest transmission speeds on a given bandwidth. {\displaystyle \Delta \theta } Because laser operates above threshold only for t ≥ td, the actual starting time of the laser output optical pulse is at t = td. Figure 23.18c shows that the frequency response of the RSOA has smooth roll-off characteristics with no relaxation oscillation peak or spectral zero, indicating that it is well suited for the electronic equalization technique based on the feed-forward equalizer (FFE) and the decision-feedback equalizer (DFE) [206]. On the other hand, off-the-shelf passive elements are widely available in the optical spectrum, allowing for an uncomplicated steering of the light beam. This performance enhancement is achieved at the cost of the increased complexity. This result indicates that it is possible to realize the high-speed colorless light source operating at 10 Gb/s cost-effectively by using the RSOA and inexpensive package. The modulation index can be classified as low (less than 1), medium (1 to 10) and high (greater than 10). Of course, the permittivities of the constituent materials also directly influence the operation. Amplitude Modulation is not only the approach of modulating the signal, but there also exist many other kinds of modulation techniques. Because the optical loss is low, the optical propagation constant may be written as ß0 = N0k0, in which case N0 is the effective optical index and k0 is the optical wave number. Like any form of shift keying, there are defined states or points that are used for signalling the data bits. Carrier-dependent refractive index in a semiconductor laser is the origin of both adiabatic chirp and transient chirp. This is commonly referred to as relaxation oscillation. (4.5) to the given m (t) yields the PM wave which is plotted in Figure 4.2 b for A c =1 volt. If m(t) is the message signal to be transmitted and the carrier onto which the signal is modulated is. Note that MU-MIMO works only in the downlink direction, from AP to client station, in the VHT specification. The technology is used in telecommunications, radio broadcasting, signal processing, and computing.In analog frequency modulation, such as radio broadcasting, of an audio signal representing voice or music, the instantaneous frequency deviation, i.e. There are two common forms of angle modulation, namely, phase modulation and frequency modulation. The bandwidth requirement of angle modulated waveforms can be obtained depending upon modulation index. Phase modulation (PM) is a modulation pattern for conditioning communication signals for transmission. Optical setup of a red, green, and blue (RGB) laser transmitter used for visible-light communications (VLC) of Gbps data rates [3]. Rate Eq. Phase Modulation(PM) is the modulation technique in which carrier phase varies based onanalog baseband information signal to be transmitted using wireless device.If a constant amplitude as well as constant frequency sine wave carrieris given to the phase shifter the output is phase modulated signal.Phase modulation is referred as indirect frequency modulation due to the fact thatphase modulation produces frequency modulation.The effect of variation in amount of phase shift is proportion… modulates the phase - the greater m(t) is at a point in time, the greater the phase shift of the modulated signal at that point. Techniques have been developed to control the bandwidth, allow shifting in the time and frequency domains, and perform mode-selective beam-splitter-like transformations.
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