11. MARCO LEGAL
13.2. FASES DEL TRABAJO
The small size, high stability, low cost and direct electrical pumping increases the range of applications of SMLLs. Moreover the ease of monolithic integrability of the SMLLs with other passive or active opto-electronic components such as modulators, multiplexers, filters and semiconductor optical amplifiers allow highly functional monolithically integrated photonic circuits. The potential applications of SMLLs are presented in the following sub- sections.
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2.4.1 Optical Communication Systems
Perhaps one of the most important areas of applications of SMLLs generating short optical pulses is the future optical fiber communication, especially operating at wavelengths around 1.3 µm and 1.5 µm. SMLLs producing short optical pulses are used as pulse sources for WDM and OTDM systems [17]. To exploit the spectral properties of SMLLs, the output from these devices can be used as a multi-wavelength source in the WDM systems by using a narrow band spectral filtering to separate an individual locked longitudinal mode [18]. SMLLs are also suitable pulse sources for the OTDM systems due to their properties including short pulse generation, high pulse repetition rates and low noise performance. In OTDM systems, as mentioned earlier, the output of a SMLL operating at X-GHz repetition frequency is split into N-channels that are delayed, modulated and then recombined to form a NxX Gbit/s signal [19]. The de-multiplexing of high speed signals in the OTDM systems is more challenging because both the clock and data need to be recovered. SMLLs have been successfully demonstrated for all-optical clock recovery [20, 21]. SMLLs also have the potential to generate high quality millimetre-wave optical signals for fiber radio transmission which could be used for personal communication systems and distribution of signals for satellite antennas [22].
2.4.2 Non-Linear Optical Effects
Short optical pulses with high peak power generated by the SMLLs can be used to obtain non- linear effects. To obtain non-linear effects in non-linear media, very high optical intensities (~ 1 GW/cm2) are required [23]. In order to get very high optical intensities, the optical pulses from SMLLs are focussed into small spots (~ 10 µm) by using a combination of lenses [24]. Second harmonic generation (SHG) is a common application of the non-linearities in which high intensity optical pulses interacting with non-linear material form photons with twice frequency and half wavelength of the initial photons. The second harmonic generation can be used to make ultraviolet, blue and green lasers [25]. SHG has also various applications in non- linear microscopy [26]. T. Yoda et al., reported high efficiency SHG by using externally amplified pulses at operating wavelength of 1550 nm [27].
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2.4.3 Optical Sampling
A common way to characterise an optical data signal is using a photodetector and an oscilloscope. However, the overall bandwidth of the fastest available photodetector allows the measurement of data signals only down to several pico-seconds pulse duration. Changing the sampling process from electrical to optical domain would increase the bandwidth of the measurement process [28]. Sources producing short optical pulses are required for optical sampling. The temporal resolution of the optical sampling measurement is determined by the sampling pulse width and the timing jitter between the measured optical data pulses and sampling pulses. Thus, the pulse source used for sampling is the most important part of the measurement system [29]. The short optical pulses emitted from the SMLLs are attractive sources for optical sampling of other short events in time that could be optical, electrical, chemical or biological. Examples of optical sampling are analogue to digital converters (ADCs) for probing of ultrafast electrical signals and pump-probe measurement for probing a medium response to another more powerful signal [30]. The potential of short optical pulses emission at high repetition rate from the SMLLs make them the most suitable choice for sampling at higher sampling rates.
2.4.4 Terahertz Radiation Generation
The terahertz (THz) region of the electromagnetic spectrum is of interest due to its non- ionizing properties, higher resolution than microwave radiations and low absorption in many materials. The main applications of the THz signals include medical imaging, spectroscopy and security [31-33]. Semiconductor lasers are widely used for THz signal generation. THz radiation can be generated simply via a photomixer by heterodyning two continuous-wave (CW) single mode lasers with a wavelength difference corresponding to desired beat frequency [34]. THz radiation could be also generated by using short optical pulses from lasers. The optical pulses from the lasers are incident on a photoconductive emitter, electron and hole pairs are generated in the semiconductor material. The charge carriers are then accelerated by a bias voltage. The resulting transient photocurrent is proportional to this acceleration and radiates at THz frequencies [35]. Terahertz photoconductive switch based on InGaAs for the pump wavelength of 1.55 μm have been reported [36]. Nowadays, the development of ultrafast photoconductive emitters made it possible to produce practical power levels of THz radiation. Another approach to convert short optical pulses into THz radiation is
Chapter 2 Background
- 14 - based on the emission of optical rectification inside a non-linear crystal. More details of this process are given in [35]. MLLs emitting high output power are required to generate THz radiation effectively and to get sufficient power in the THz components [24, 31].