Optical bistability and its applications A Review

Transcription

1 International Journal of Technical Innovation in Modern Engineering & Science (IJTIMES) Impact Factor: 5.22 (SJIF-2017), e-issn: Volume 4, Issue 7, July-2018 Optical bistability and its applications A Review Shashank Verma 1, Dr. Divya Dhawan 2 1 Department of Electronics and Communication Engineering, Punjab Engineering College (Deemed to be University) Chandigarh, India, shashankverma94@gmail.com 2 Department of Electronics and Communication Engineering, Punjab Engineering College (Deemed to be University) Chandigarh, India, divyadhawan@pec.ac.in. Abstract This paper presents a review of some applications and some effects like self-pulsation, switching and mode hopping that occur due to bistability in optical devices, the applications discussed are important contributions in the area of optical computing and realization of all optical Analog to Digital Converter (ADC). In this paper molecular bistability (absorptive and dispersive), intrinsic and hybrid bistability are discussed, although the applications reviewed are based on intrinsic bistability. Keywords Optical bistability, Self-pulsation, Mode hopping, Q-switching, optical hysteresis. I. INTRODUCTION Since the discovery of optical bistability in 1974, it has been observed in many materials, including tiny semiconductor etalons[1]. Devices that exhibit bistable properties consists of a non-linear medium contained in an optical resonator but excited only by incident coherent light. In general, a system is said to be bistable if it produces two output states for the same value of input, the input output characteristics are defined by hysteresis loop as shown in fig 1. Fig 1. input output characteristics of an optical bistable system [1]. The bistable systems can be classified into two sections either dispersive and absorptive or intrinsic and hybrid[1][2]. The bistability is a result of the feedback received by the optical devices. If the feedback occurs due to intensity dependent absorption or by refractive index, the system is said to be absorptive/dispersive bistable[1][3]. It s usually hard to distinguish between absorptive and dispersive bistability as both the phenomenon occurs simultaneously due to feedback. Whereas, in an intrinsic system, the bistability is caused by direct interaction of light with device matter, and when the intensity dependence arises from an electrical signal the system is said to be hybrid[1]. Among all the devices one such device exhibiting bistability is laser. For bistability to occur, feedback is an important requirement[4]. The laser section/gain medium is placed in an optical resonator, where the partially reflecting mirrors becomes the source of feedback. Also, in semiconductor lasers, facet reflectivity (cleaved facets) is a huge factor as it contributes in feedback, one such laser is Fabry-Perot laser[3]. Apart from Fabry-Perot laser, Distributed Feedback (DFB) laser employing a grated medium also exhibits the bistability under controlled conditions. Experimentally, the optical bistability can be observed in a DFB laser by injecting continuous wave light in it. The injected light must have different wavelength (more than 1nm away) from the operating wavelength of to avoid interaction with DFB grating [5]. With the change in input power levels, the uniformity of carrier density is affected. The increase in input power above threshold level can cause the carrier density to lose uniformity across the laser section and hence causing the laser action to discontinue. This phenomenon is known as Spatial hole burning effect (SHB)[6]. IJTIMES-2018@All rights reserved 69

2 II. APPLICATIONS BASED ON OPTICAL BISTABILITY The applications to be discussed includes an all optical square wave clock generator, an all optical analog comparator, an optical Schmitt trigger and an all optical flip flop based on a single distributed feedback laser apart from the abovementioned applications the digital signal regeneration, optical de multiplexing[13] worth noting. A. All optical square wave generator The clock is used to drive basic electronic gate, in the same way it s also required in photonic integrated circuits for timing. M. R. Sayeh et al. presented an all optical square wave generator [14] which is based on ring lasers that consists of semiconductor optical amplifier (SOA), as an active medium. Fig 2: A proteretic bistable device [14]. The complete setup consists of 3 ring lasers as shown in fig. 2, out of which 2 unidirectional ring lasers are coupled so that they exhibit bistable properties so that they constantly move in descending and ascending-switching threshold regions (see fig. 1) resulting in square wave generation. The frequency of the clock can be changed by adjusting the length of ring lasers and duty cycle can be adjusted by changing the bias current of the third SOA. B. All optical analog comparator The all optical comparator (AO comparator) presented by P. Li et al. [15] utilizes a multiple quantum well distributed feedback laser (MQW-DFB laser), in their paper they suggested two models in which the laser is driven by a continuous wave laser or by Mode locked laser modulated by sinusoidal wave of 500MHz and 1.25 respectively as shown in fig 3. The laser section (I.e. Multiple quantum well DFB laser) acts as an optical comparator followed by a band pass filter (used to filter noise). Fig 3: Schematic of an all optical analog comparator, EOM: electro optic modulator and BPF is band pass filter The laser is based on the optical hysteresis and the AO comparator exhibits excellent comparator performance parameters like step like transfer function, large extinction ratio (40 db), low threshold current (26 mw), and fast operating frequency. AO comparator is very important in realization of all optical ADCs and can remove the electrical bottleneck. IJTIMES-2018@All rights reserved 70

3 C. Optical Schmitt Trigger H. Sarafraz et al. presented an optical Schmitt trigger[16] which is based on ring lasers consisting of SOAs, the model discussed involved two ring lasers, in the absence of input ring 1 dominates over ring 2 as its injection current is higher, the gain spectrum is chosen to be parabolic having peak at other loop s wavelength so that each loop can suppress the other loop power, however when the input is applied, that is a continuous wave light modulated by 1 sinusoidal wave the first SOA deviates from its resonating wavelength hence the second ring laser gets activated hence generating a Non inverting output. D. All optical filp flop based on a single distributed feedback laser This application works similar to all optical analog comparator as it also uses DFB laser, in one state the laser is lasing and the CW light injected is weakly amplified due to gain clamping, in the other state the injected CW light gets highly amplified resulting in non-uniform carrier density (spatial hole burning effect), thus the threshold increases and the laser turns off[5]. For operation of DFB laser as a flip flop the bistable properties are exploited using positive pulses (200 ps pulses of 500fJ) to switch between two states, switching to non-lasing state from lasing state is done by injecting a strong short pulse from the same side as of CW light this will make the carrier density non-uniform hence laser will turn off. To restore the lasing, operation the laser a short pulse is fired from the opposite direction. The setup used for this application is similar to fig. 3. The contrast ratio and repetition rate achieved is 32 db and 1.2. Suzuki et al. did the analysis of a laser diode having a saturable absorber in its cavity as an optically triggered opticallogic-gate[17]. From their results it is concluded that implementation of high speed logic optical-logic-circuits can be done with laser diodes having a saturable absorber, and the ultimate limitation in the response time is set by the round trip time of the light in the laser diode cavity under strong triggering pulse. E. Digital Signal Regeneration The bistable properties of a laser diode can be utilized to work as digital signal representation device, K. Nonaka et al. developed a wavelength converter that can work as a digital signal regeneration device due to its threshold amplification characteristics, the device can be used to regenerate signals that have low extinction ratio, and are affected with deformation and power fluctuation [18], the device consists of two waveguides one is the main waveguide for laser output and an orthogonally crossed sub waveguide for the input section. The sub waveguide is responsible for the input amplification and link it to the saturable absorption region of the main cavity, the thresholding characteristics helps in suppressing the noise level and improving the extinction ratio (17 db) for 622 Mb/s and 1-Gb/s bit rates. F. Optical de-multiplexing The direct de-multiplexing of optical signals and wavelength conversion is demonstrated by Nonaka et al. [19] by using bistable laser diode, full scheme is shown in fig 4. The multiplexed signals are distributed to WDM local area networks where bit rates of the signals are reduced and wavelength conversion takes place, wavelength conversion can take place with two mode bistability, Fig 4: optical de-multiplexing scheme [17] where the active section of the laser is excited by an external CW laser and the bistability between these two modes occur due to cross gain saturation and the saturable absorbers if the injection current is within the hysteresis loop[20]. Wavelength conversion. High speed optical grating and bit rate conversion are all required simultaneously. To demultiplex NRZ signals and perform wavelength conversion simultaneously the photonic device must also have the function for gate operation, memory and wavelength conversion[19], the clock pulses for voltage control part is provided by the trigger using a pin diode. IJTIMES-2018@All rights reserved 71

4 (Author, year)/type of device Sun et al. (United states patent)1991/jacobs et al.2004, Optically clocked track and hold Ma et al., 2003/Electronic ADC Chi, H. & Yao, J, 2009/ Optically sampled and Quantised ADC Ikeda, K. et al, 2005/ All optical quantizing and encoding scheme Stigwall & Galt 2005/ optical quantizing scheme. Lowery A.J. 1987/ Laser source. Input: 1, sampling speed: GS/s TABLE 1 PAPERS REVIEWED Parameters Inference Technology Phenomenon of Bistability MLL used as clocking Optical Sampling, device, limitations Electronics recognized: slow turn on quantization and off times, not good for encoding high freq. applications ENOB=4 Input bandwidth= 40 ENOB= quantization levels 3-bit A/D convertor Speed of 10GS/s ENOB=2.6 Input bandwidth = ns transient and fast speed operation. 6 times better than other electronic ADCs 3 quantization tech. discussed, MZMs with similar electrode lengths removes the need of different bias voltage. Optical sampling technique useful for further research is studied. Use of fiber non linearities, to increase speed of operations. Optical sampling, quantizing is done but thresholding with electronic comparators Transmission line laser model used with fast transients found. Analysis of mode hopping effect. Analysis of selfpulsation. Bronius Šaulys et al, 2010/Fabry-Perot Laser. M. Ganesh Madhan et al 1999/ QW bistable laser diode Hitoshi Kawaguchi 1981 Analysis of bistable nature with varying temperature. Hitoshi Kawaguchi 1997 Applications of bistable lasers are discussed. Yosef Ehrlichman et al Step like Used Mach-Zehnder 2013/ Integrated Electrical/Optical interferometer as Photonics thresholder response thresholder Yong deok Jeong et al 2006/ all optical flip flop Pu Li et al, 2016/ All optical analog comparator Pu Li et al, 2017/All optical analog comparator Rising and falling edge times 0f 50ps, set and reset pulse power less than -9dBm Extinction ratio :40db Input bandwidth =2.5 Not mentioned Fabry-Perot laser diode used (injection locked) Mode locked laser used for sampling, QWS-DFB laser for quantization and encoding. Response time improved thus comparator exhibits exact rectangular step like transfer function. Electronic Optical Sampling and Quantizing, Electronic comparator used Photonics quantization introduced, electronic thresholding. No optical comparators Switching. Full working optical comparator. Full working optical comparator Mode hopping. Self-pulsations. Use of Optical hysteresis and switching., Nonlinear fiber Ring resonator used. IJTIMES-2018@All rights reserved 72

5 III. CONCLUSIONS In this paper the concept of optical bistability, it s types and some applications based on it are discussed. It s fascinating that laser section s mode of operation can be changed by merely adjusting the bias current. All the applications discussed utilizes the switching characteristics dependent on the ascending and descending threshold as per hysteresis. The extinction ratio achieved is as large as db with. In this paper laser mode of operation like self-pulsation, mode hopping and switching is discussed. The phenomenon of optical bistability is very useful in the realization integrated photonics circuits and basic components like all optical ADCs, Flip flops and switching circuits, creating a way to eliminate electronic bottleneck and providing highest ever data rates. REFERENCES [1] H. Gibbs, Optical Bistability, Controling Light with Light, 1st ed. Academic Press Inc., [2] R. Reinisch and G. Vitrant, Optical bistability, Prog. Quantum Electron., vol. 18, no. 1, pp. 1 38, [3] T. Vivero, J. M. Rivas-Moscoso, A. P. Gonzalez-Marcos, and J. A. Martin-Pereda, Dispersive Optical Bistability in Quantum Wells With Logarithmic Gain, IEEE J. Quantum Electron., vol. 46, no. 8, pp , [4] S. a Collins and K. C. Wasmundt, Optical Feedback and Bistability: a review, Opt. Eng., vol. 19, no. August 1980, [5] K. Huybrechts, G. Morthier, and R. Baets, Fast all-optical flip-flop based on a single distributed feedback laser diode, Opt. Express, vol. 16, no. 15, pp , [6] W. Fang, A. Hsu, S. L. Chuang, T. Tanbun-Ek, and A. M. Sergent, Measurement and modeling of distributedfeedback lasers with spatial hole burning, IEEE J. Sel. Top. Quantum Electron., vol. 3, no. 2, pp , [7] M. Ganesh Madhan, P. R. Vaya, N. Gunasekaran, "Self-pulsation in a quantum-well bistable laser diode," Proc. SPIE 3666, International Conference on Fiber Optics and Photonics: Selected Papers from Photonics India '98, (28 April 1999). [8] H. Wenzel, U. Bandelow, H. J. Wünsche, and J. Rehberg, Mechanisms of fast self pulsations in two-section DFB lasers, IEEE J. Quantum Electron., vol. 32, no. 1, pp , [9] R. Schatz, Longitudinal Spatial Instability in Symmetric Semiconductor Lasers due to Spatial Hole Burning, IEEE J. Quantum Electron., vol. 28, no. 6, pp , [10] U. Bandelow, H. J. Wunsche, and H. Wenzel, Theory of selfpulsations in two-section DFB lasers, IEEE Photonics Technol. Lett., vol. 5, no. 10, pp , [11] B. Šaulys, J. Matukas, V. Palenskis, S. Pralgauskait, J. Vyšniauskas, and B. Saulys, Analysis of mode-hopping effect in fabry-perot laser diodes, Radar Wirel., vol. 9, no. Iii, pp. 3 6, [12] VPI Photonics component maker, user s manual, no. version 9.0, [13] H. Kawaguchi, Bistable laser diodes and their applications: State of the art, IEEE J. Sel. Top. Quantum Electron., vol. 3, no. 5, pp , [14] N. Davoudzadeh and M. R. Sayeh, All-optical square-wave clock generation, in 2015 IEEE Photonics Conference (IPC), 2015, pp [15] P. Li, X. Yi, X. Liu, D. Zhao, Y. Zhao, and Y. Wang, All-optical analog comparator., Sci. Rep., vol. 6, no. July, p , [16] H. Sarafraz and M. R. Sayeh, Analysis on optical bistability parameters in photonic switching devices, vol. 55, no. 6, [17] Y. Suzuki, J. Shimada, and H. Yamashita, HIGH-SPEED OPTICAL-OPTICAL LOGIC GATE FOR OPTICAL COMPUTERS, Electron. Lett. IEEE, no. 4, pp. 3 4, [18] K. Nonaka, Y. Noguchi, H. Tsuda, and T. Kurokawa, Digital Signal Regeneration with Side-Injection-Light- Controlled Bistable Laser Diode as a wavelength converter, IEEE Photonics Technol. Lett., vol. 7, no. 1, pp , [19] K. Nonaka and T. Kurokawa, Simultaneous time- and wavelength-domain optical demultiplexing of NRZ signals by using, vol. 31, no. 21, pp , [20] M. Takenaka, M. Rabum, and Y. Nakano, Regenerative wavelength conversion using optical flip-flop outputs of multimode interference bistable laser diodes, Conf. Proc. - Int. Conf. Indium Phosphide Relat. Mater., vol. 2005, pp , IJTIMES-2018@All rights reserved 73