Mode-locked Hybrid Soliton Pulse Source Research Articles

RIN of Mode-Locked External Cavity Lasers Utilizing a Sinusoidally Chirped Fiber Bragg Grating

The relative intensity noise (RIN) of a mode-locked hybrid soliton pulse source (HSPS) utilizing a sinusoidally chirped fiber Bragg grating (FBG) is investigated using the time-domain solution of coupled wave equations and rate equations that include both spontaneous noise and carrier density noise. The effect of some laser diode and grating parameters on the RIN of the HSPS is also described. The numerical results indicate that most parameters affect the system operation even in the absence of noise. RIN reduction at the fundamental frequency is possible in a mode-locked HSPS utilizing a sinusoidally chirped, Gaussian-apodized FBG when compared with an HSPS with linearly chirped gratings. Furthermore, it is also shown that an HSPS utilizing a sinusoidally chirped FBG produces shorter pulses with and without noise than linearly chirped gratings.

Sinusoidally chirped fiber Bragg grating for mode-locked application

In this study, a mode-locked hybrid soliton pulse source (HSPS) utilizing sinusoidal chirped fiber Bragg grating (FBG) is reported for the first time, using the time-domain solution of coupled wave equations and rate equations. The sinusoidal chirped FBG provides a wider bandwidth by adjusting the reversion coefficient or chirp rate even if the FBG length is short. Numerical results also indicate that an HSPS-utilized sinusoidal chirped FBG produces shorter pulses in the 25−72 ps range, whereas the pulses range from 31 to 97 ps for a linearly chirped tanh apodized grating, and from 30 to 80 ps for a linearly chirped Gaussian apodized grating, along with an increase in the mode-locking frequency range.

Effect of Grating Parameters on Mode-Locked External Cavity Lasers

This paper deals with numerical solutions of actively mode-locked fiber grating semiconductor lasers using a time-domain solution of coupled wave equations and rate equations. Simulation of linearly chirped tanh apodized fiber Bragg grating (FBG) utilized in hybrid soliton pulse source (HSPS) shows an extreme increase in the mode-locking frequency range of HSPS. Our model predicts transform-limited pulses over a frequency range of 1.6 GHz (1.8-3.4 GHz) for this grating around a system operating frequency of 2.5 GHz, with a pulsewidth of 46 ps required for a practical soliton transmission system, whereas it ranges to about 1.3 GHz (2-3.3 GHz) for the linearly chirped raised-cosine flat top and 850 MHz (2.1-2.95 GHz) for linearly chirped Gaussian apodized. Furthermore, in this study, the effects of FBG parameters, such as peak reflectivity, grating length, grating chirp, and modulation index, on output of mode-locked HSPS are also described for the first time. The numerical results indicated that although pulsewidths decreased with the increase in grating chirp, shorter grating lengths gave shorter pulses, and the modulation index and peak reflectivity of the grating did not significantly affect the pulsewidths.

Theoretical model of the mode-locked hybrid soliton pulse source

A theoretical model for the mode-locked hybrid soliton pulse source HSPS is developed by using a time domain solution of coupled- mode equations and rate equations. Numerical simulations show that grating must be both linearly chirped and apodized to be used in HSPS systems with a wide mode-locking frequency range 2.1 to 3 GHz and transform-limited output pulses. © 2007 Society of Photo-Optical Instrumentation tively. HSPS utilizing a uniform grating Bragg reflector inte- grated into a fiber external cavity has been shown to oper- ate close to the required frequency with transform-limited pulses and very high output powers. 1 However, this device worked well only under specific operating conditions and showed spectral instabilities when operating parameters were changed. Utilizing the device with a linearly chirped grating has overcome the spectral instability problem of the earlier device. An interesting feature of these devices is the extremely wide operating repetition frequency range, which can be enhanced by using chirped gratings, as reported in Refs. 2 and 3. In Ref. 3, measured results showed that transform-limited pulses are generated over a repetition fre- quency range of 850 MHz using a linearly chirped Gauss- ian apodized fiber Bragg grating FBG. However, in this paper, the theoretical model of the mode-locked HSPS with linearly chirped Gaussian apodized FBG is described. Ob- tained results indicate that transform-limited pulses are generated over a wide tuning range of 900 MHz for linearly chirped Gaussian apodized FBG, and this grating is a good candidate to be used in soliton pulse propagation systems. All results have been found to fit well compared with the experimental results in the literature. 3 2 Model of the HSPS The structure of the HSPS utilizing linearly chirped FBG considered in our analysis is shown in Fig. 1. The geometry is the same as the one reported in Refs. 3-7. The HSPS system is made up of a multiquantum well MQW semi- conductor laser, a fiber, and a FBG. One facet of the diode is high reflectivity HR-coated, and the other is antireflec- tion AR-coated. The light from the AR-coated facet is coupled to the FBG reflector. The field in this system trav- els between the HR-coated laser end and the effective cav- ity length of the grating. The output power is taken through the grating. The mode-locked HSPS model is based on a time- domain solution of the coupled-mode equations. Assume that the longitudinal, effective-refractive index variation of the Bragg grating is given as

Intensity Noise in Mode-Locked External Cavity Lasers

Effect of spontaneous and carrier density noise on a mode-locked hybrid soliton pulse source (HSPS) utilizing linearly chirped fiber Bragg grating is investigated by using coupled mode-equations. It is found that transform-limited pulses are generated from mode-locked HSPS over a wide tuning range around the fundamental mode-locking frequency without noise and with low level of both spontaneous and carrier densiy noise sources. However, mode-locking is difficult to achieve for large value of spontaneous coupling factor, linewidth enhancement factor, gain saturation parameter and some value of rf and dc currents because of increasing noise. High noise introduces a noise peak in the relative intensity noise (RIN) spectrum making transform-limited pulses unobtainable. Hence, the proper mode-locking range where transform-limited pulses are generated reduces.

Effect of Spontaneous and Carrier Noise on Mode-Locked Hybrid Soliton Pulse Source

Noise characteristics of mode-locked hybrid soliton pulse source (HSPS) where fiber Bragg grating is used as external cavity is presented. Model is based on time-domain solution of coupled-mode equations including spontaneous noise. Relative intensity noise (RIN) is calculated by using numerical solution of these equations. It is shown that inclusion of carrier density noise in the rate equations is necessary for the accurate analysis of noise in mode-locked lasers. It is also found that transform-limited pulses are not generated over a wide tuning range around the fundamental mode-locking frequency with spontaneous and carrier density noise sources if noise level is high. Therefore mode-locking range where transform limited pulses are obtained reduces.

Mode-locking characteristics and relative intensity noise reduction in hybrid soliton pulse source

The noise and mode-locking phenomena of a hybrid soliton pulse source (HSPS) utilizing different fiber Bragg gratings (FBG) with or without linear chirp are described. The HSPS is modeled by a time-domain solution of the coupled-mode equations including spontaneous emission noise, and relative intensity noise ( RIN ) is calculated using numerical solutions of these equations. It is found that transform-limited pulses over a wide tuning range around the fundamental mode-locking frequency are generated from mode-locked HSPS with linearly chirped uniform and Gaussian apodized FBGs. Mode-locked pulses that are not transform-limited are generated over a wide tuning range from HSPS with Gaussian apodized and uniform FBGs. These gratings can give transform-limited pulses only over a limited tuning range around the fundamental mode-locking frequency. It is also found that RIN reduction is possible for the mode-locked HSPS by selecting a suitable apodization function such as Gaussian and linear chirp rate.

Effect of intensity noise on mode-locked hybrid soliton pulse source

Effect of spontaneous noise on mode-locked hybrid soliton pulse source (HSPS) utilizing linearly chirped fiber Bragg grating is described by couple-mode equations including spontaneous emission noise. Relative intensity noise (RIN) of mode-locked HSPS is given for different linewidth enhancement factors and spontaneous coupling factors to show the effect of these parameters on RIN and output pulse of HSPS. For large values of these parameters, proper mode-locked is difficult to achieve because of increasing RIN and therefore transform-limited pulses are not generated over a wide frequency range whereas transform-limited pulses are generated over a wide frequency range with low RIN. RIN is also extremely sensitive to the r.f. and d.c. currents.

RIN in mode-locked external cavity semiconductor lasers

The relative intensity noise (RIN) of a mode-locked hybrid soliton pulse source (HSPS) utilising a linearly chirped uniform fibre Bragg grating, is investigated, taking into account spontaneous noise, by using coupled-mode equations. It is shown that low noise does not affect the operation of device and therefore transform-limited pulses are generated over a wide tuning range around the fundamental frequency. However, for large values of linewidth enhancement factor, spontaneous coupling factor and gain saturation parameter, transform-limited pulses are not obtained over a wide tuning range because of increasing noise. Hence, the proper mode-locked range reduces. It is also shown that RIN reduction is possible by using linearly chirped uniform gratings as compared to unchirped gratings.

Mode-locked hybrid soliton pulse source with extremely wide operating frequency range

The authors report a mode-locked pulse source with extremely wide operating frequency range and very stable operation, through the use of a long, linearly chirped Bragg reflector as the output coupler integrated in a fiber external cavity. A 1.55 mu m strained MQW laser diode is used, with one facet high reflectivity (HR) coated for improved cavity Q, and the other antireflection (AR) coated to allow coupling to the external cavity and suppress Fabry-Perot modes. Near-transform-limited pulses are obtained over a frequency range of 700 MHz around a system operating frequency of 2.488 GHz, with pulsewidths of 50 ps, as required for a practical soliton transmission system. >