RF OPTOELECTRONIC OSCILLATOR USING A DIRECTLY MODULATED SEMICONDUCTOR LASER AND A FIBER OPTICAL RING FILTER H. E. Kotb,1 A. M. E. Safwat,2 H. Boghdady,3 and D. A. M. Khalil4 1 Transmission Department, National Telecommunication Institute, Nasr City, 11768, Cairo; Corresponding author:
[email protected] 2 Electronics and Communication Engineering Department, Faculty of Engineering, Ain Shams University, Abassia, 11517, Cairo 3 Transmission Department, National Telecommunication Institute, Nasr City, 11768, Cairo 4 Electronics and Communication Engineering Department, Faculty of Engineering, Ain Shams University, Abassia, 11517, Cairo
Figure 1 The proposed optoelectronic oscillator using internal modulation technique and all optical RF filter
Received 31 May 2008 ABSTRACT: In this article we present an RF optoelectronic oscillator based on directly modulated laser diode and an all optical RF filter. The system is modeled and simulated using standard simulators. Using commercial modules and standard single mode fiber, the implemented system shows an RF oscillation with 9.8 dBm at 1.1 GHz. We also show that the use of an all optical RF filter allows a sidemode suppression ratio more than 50 dB without affecting the signal purity. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 470 – 475, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24069 Key words: optoelectronic oscillator (OEO); phase noise; quality factor; all-optical filter
1. INTRODUCTION
Recently optoelectronic oscillators (OEOs) have attracted increasing interest because of their spectrally pure oscillations. This spectral purity is mainly because of the use of a long optical fiber delay line in a feedback loop, which includes a laser source, an external modulator, and a photodetector [1–3]. The optical fiber cable behaves as a low loss long storage element, which reduces greatly the phase noise down to ⫺140 dBc/Hz at 30 KHz frequency offset when its length is ⬃240 m [1, 2]. These interesting results are very attractive for applications where a very high signal to noise ratio and high frequency stability are required. OEOs have two practical difficulties: First, the long fiber results in a small free spectral range (FSR), (in the above example the FSR is 834 KHz). Therefore a tuned very narrow-band RF bandpass filter, which is quite difficult to realize at high frequencies, is required. This problem was overcome by using two feedback loops; one has a short length to increase the FSR and the second is much longer to act as ultra high energy storage element. The resulting oscillator has a highly pure spectral line with large FSR [4, 5]. In this dual-loop configuration, the open loop gain of each feedback loop may be less than unity as long as the combined open loop gain of both loops is larger than unity. In addition, each oscillation mode with frequency fosc must add up in phase after each round trip around both loops [4, 5]. However, this parallel dual-loop OEO sacrifices the high quality factor (Q) produced from the long loop fiber. The total Q is averaged between the long high Q loop and the short low Q loop so the phase noise increases compared with the single-loop long-fiber OEO [6, 7]. The Q factor drawback was cured by using a master OEO with very low phase noise to lock the frequency of slave OEO with high phase noise in an injection locking technique [7]. This technique adds extra components to the oscillator which increases its complexity. In this
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work we propose insertion of an all-optical filter in the feedback loop to allow for the modes selection with a much simpler technique. We thus, propose a new configuration of the OEO in which the external modulator is replaced by the direct modulation of the laser diode and, the RF signal is filtered in the optical domain instead of the RF domain. This overcomes the need of a high Q narrow-band RF filter. The results obtained from this system are very encouraging as they demonstrate that the added optical filter does not affect the OEO phase noise in the band of measurement. In addition, the use of the laser diode allows controlling the output RF frequency through the control of the laser diode DC current. Frequency tuning of the oscillator is also demonstrated in this paper. The article is organized as follows, the basic theory of the proposed oscillator and the conditions for stable oscillation are presented in Sections 2 and 3, respectively. The tuning method is discussed in Section 4. The simulated results are demonstrated in Section 5. Section 6 shows the experimental demonstration of the proposed OEO. 2. THEORY
The schematic diagram of the proposed OES is shown in Figure 1, The modulated optical signal is detected by the photodetector after passing through an all optical RF filter. After detection, the electrical signal is amplified and fed back to the input port of the laser driver. The open loop block diagram of the OEO is shown in Figure 2 where each component is described by its transfer function. 2.1. Laser Diode Model The small signal transfer function of the directly modulated laser diode, HL(), is the ratio between the small signal output optical power (pm) and the small signal input modulating current (im). It can be considered as a second order low pass filter, with the following expression [8]: H L共兲 ⫽
Figure 2
Pm A ⫽ 2 im r ⫹ i␣ ⫺ 2
Open loop block diagram of the proposed OEO
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(1)
Figure 3
Small signal model of the photodetector
with A ⫽
冉 冊
Io h ⫺1 2e⌫o spph Ith
␣⫽
and r2 ⫽
Io spIth
(2)
(3)
冉 冊
1 Io ⫺1 spph Ith
(4)
where h is Planck’s constant, is the optical signal frequency, ⌫o is the optical gain confinement factor, sp is the carrier spontaneous life time, ph is the photon life time, Io is the laser diode dc bias current, Ith is the laser diode threshold current, and r is the laser diode relaxation frequency. This small signal model is useful in getting analytical expressions for the amplitude and phase conditions of the oscillator. However, for the oscillator operation, a large signal model is necessary to account for the laser nonlinearity. Such large signal model is built using the laser rate equations, implemented directly in the model. The simplest laser rate equations used in the model are given as: dN J N ⫽ ⫺ ⫺ go共N ⫺ Ntr兲S dt ed sp
(5)
dS N S ⫽ ⌫g o共N ⫺ Ntr兲S ⫺ ⫹  dt ph sp
(6)
where: N is the Carrier density, S is the Photon density, J is the current density, go is the optical gain parameter, and  is the spontaneous emission factor. 2.2. Photodetector Model The photodetector converts the input optical power into current with a ratio () that represents its responsivity. Its small signal
Figure 4
All optical RF filter based on optical fiber ring resonator
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Figure 5 Frequency characteristics of Fabry Perot filter; (a) Frequency response, measured (dotted line) and simulated (solid line), (b) Phase response in degree
model can be simply presented by its junction capacitance (Cd).. Combining this capacitance with the bias resistance (RL), and the amplifier input impedance (Rin, Cin), as shown in Figure 3, the small signal transfer function of the photodetector (Hd() ⫽ vo/pm) is thus [9]: H d共兲 ⫽
0 RL//Rin ⫽ Pm 1 ⫹ i共RL//Rin兲共Cin ⫹ Cd兲
(7)
2.3. All Optical RF Filter The insertion of the all optical RF filter in the oscillator forward loop changes the multimode oscillation into a single mode one as the oscillator phase condition is controlled by the phase response of the filter [10]. The possible oscillation modes are the filter modes; however, one of the filter modes is coincided with the resonance frequency (fr), so it will have the maximum gain while the loop gain of other modes will be less than unity. The diagram of all optical RF filter is shown in Figure 4. It is Fabry Perot ring resonator which consists of an optical coupler that has a coupling ratio k, input fiber of length Lext and a second fiber
Figure 6
Laser diode and photodetector modulation response
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Figure 7 Laser diode small signal amplitude response versus frequency plotted for different Io.
of length Lf that connects Port 2 and Port 3. The transfer function of the RF signal modulating the optical signal is [11]: k ⫹ 共1 ⫺ 2k兲exp共 ⫺ jLf兲 1 ⫺ kexp共 ⫺ jLf兲
H共 RF兲 ⫽ exp共 ⫺ jLext兲
(8)
where  is RFneff/c, neff is the effective optical refractive index and c the speed of light. The amplitude and phase responses of the filter having a coupling coefficient (k) equals to 0.5, Lf ⫽ 7 m and Lext ⫽ 7 m are shown in Figure 5. 2.4. RF Amplifier The RF amplifier in the feedback loop is required to compensate for the losses in the laser driver circuit, the photodetector circuit and the optical fiber cable. The total loss was measured and found to be equal to 17 dB at fr ⫽ 1.124 GHz (see Figure 6). The amplifier small signal gain must be larger than 17 dB. An amplifier of gain 21 dB at fr followed by a 3-dB power splitter was used. 3. OSCILLATION CONDITIONS
3.1. Amplitude Condition The small signal open loop gain (Gs) is obtained from the small signal model of the different elements in the oscillator loop. Since the loop consists of a laser diode, a photodetector and a tuned amplifier, from (1), (7), and (8) the open loop gain is: 兩G s兩 ⫽
冑共
2 r
A ⫺ 2 兲2 ⫹ 共␣兲2
⫻
RL//Rin
冑1 ⫹ 共RL//Rin兲2共Cin ⫹ Cd兲2 2
兩H共兲储Av 储Gm兩 ⱖ 1
(9)
Figure 8 Oscillation frequency versus the dc current normalized to the threshold current (Io/Ith), practical results dotted points
It should be noted that the oscillation occurs close to the laser relaxation frequency value; therefore, it is expected that the oscillation frequency will be very close to the laser relaxation frequency fosc ⫽ fr. which is verified in the simulated and practical results. 3.2. Phase Condition For oscillation to take place, the phase of the closed loop must be an integer number of 2ð. From (1), (7), and (8) this is equivalent to:
⫺ tan⫺1
冉
冊
␣ ⫺ tan⫺1 共共RL//Rin兲共Cd ⫹ Cin兲兲 ⫺ 2 2 r
(10)
⫹ 共 兲 ⫹ ⬔H共 兲 ⫽ 2m where is the group delay of the single mode optical fiber, m is an integer number, and is the RF amplifier phase shift. 4. FREQUENCY TUNING
The laser diode small signal response HL() is plotted versus frequency in Figure 7 for a laser diode with rate equation parameters shown in Table 1. The laser diode amplitude reaches a peak value at the relaxation oscillation frequency fr, which is affected by the dc current Io. Increasing Io results in a shift in the position of the peak amplitude thus a shift in the relaxation frequency as well. As the oscillation takes place very close to the peak in the loop gain, it is expected to be close to the relaxation frequency that is fosc ⬇ fr [12].
where Av is the voltage gain of the tuned amplifier, Gm is the current to voltage transducer ratio, and H() is the filter transfer function. To sustain oscillations, the small signal open loop gain (Gs) must be greater than unity. However, for the exact determination of the amplitude condition, a large signal model is required.
TABLE 1 Parameters of Laser Diode Used the Practical Experiment for OEO Frequency Tuning Spontaneous carrier life time n Photon life time ph Spontaneous emission parameter  Threshold current Optical wavelength
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0.316 ns 10.4 ps 10⫺3 5.2 mA 1.3 m
Figure 9
Block diagram describing the laser diode rate equations
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TABLE 2 Parameters of the Laser Diode Used in the Simulation Spontaneous carrier life time sp Photon life time ph Spontaneous emission parameter  Threshold current Optical wavelength
Figure 10
2 ns 0.025 ns 10⫺3 11 mA 1.3 m
Photodetector model
Since fr is a function of ratio between Io and the threshold current Ith, the oscillation frequency can be controlled by varying the dc driving current. In addition, the phase of the oscillating signal is also a function of the laser diode dc driving current given by: (tan⫺1(␣/(r2 ⫺ 2))). The control of the laser diode DC current allows thus controlling both the amplitude and the phase condition in the loop. Figure 8 shows the simulated curve of the frequency tuning performance versus the dc driving current. The laser diode rate equation parameters, extracted by frequency response measurement [13], are used to plot the frequency tuning performance of the oscillator. 4. SIMULATION RESULTS
The proposed oscillator model is tested using the standard system simulator. To determine the oscillation amplitude, the laser diode is modeled using its rate equations to account for the nonlinearity and large signal response of the laser [14]. The schematic model of the laser diode is shown in Figure 9. The parameters reported in Table 2 are used during the simulation. The Photodetector is modeled as a first order low pass filter with a bandwidth of 1.5 GHz as shown in Figure 10. An attenuator is added to model the detector responsivity, which is assumed to be 0.7 mA/mW in our case. The amplifier is modeled as a linear gain block and the delay experienced by the fiber cable is modeled by a transport delay. The OEO components are connected as shown in Figure 11. A dc current equals to 35 mA is added to the RF current at the input
Figure 11
Simulated block diagram of the proposed oscillator
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Figure 12 Oscillator output spectrum; (a) L ⫽ 110 m, (b) L ⫽ 100 m with the insertion of all optical RF filter
of the laser diode. The output optical power from the laser diode block is delayed by the transport delay and enters the photodetector. The output RF voltage from the photodetector goes to the RF amplifier where it gets multiplied by the gain Gm and converted to an RF current that returns back to the laser diode. The proposed oscillator is simulated with and without the all optical RF filter. For a fiber delay length L ⫽ 100 m there are many oscillating modes separated by 2 MHz, as shown in Figure 12(a). Inserting the all optical RF filter in the feedback loop converts the multimode oscillation into single mode oscillation, as shown in Figure 12(b). 5. EXPERIMENTAL RESULTS
The experimental setup, shown in Figure 13, consists of a laser diode transmitter module, with an RF modulation bandwidth that extends to 1.1 GHz, an avalanche photodetector, an RF amplifier that has a gain equals to 21 dB and 3-dB bandwidth of about 90 MHz with a 3-dB coupler connected to the this amplifier, a 100 m optical fiber cable, inserted in the forward loop, and all optical RF filter constructed using commercially available 3-dB optical directional coupler.
Figure 13
Experimental setup of the oscillator
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Figure 14
Measured output spectrum from the oscillator, L ⫽ 100 m
The experimental results of the OEO with an optical fiber cable that has a length equals to 100 m is shown in Figure 14 where multimode oscillation can be observed. The oscillation frequency (Fosc) is 1.116 GHz, the output power (Posc) is 6 dBm and the mode separation is 2 MHz. A magnified picture corresponding to a frequency span equals to 20 KHz around the center frequency is shown in Figure 15. The experimental results of the OEO with the all optical RF filter (with Lf ⫽ 7 m) and an optical fiber length equals to 100 m, are shown in Figure 16. Another RF amplifier of 11 dB is inserted in feedback loop to compensate the filter insertion loss. This will increase the oscillation output power. The multimode oscillation is
Figure 17 with filter
Output spectrum of the oscillator showing its spectral purity
converted into nearly single frequency oscillation at the RF frequency fosc ⫽ 1.108 GHz, with an RF power, Posc ⫽ 9.8 dBm. The higher order modes are greatly suppressed and we have at least a mode suppression ratio (ratio between the oscillation maximum and the maximum of the first highest order mode) of more than 50 dB and the first higher order mode is separated by about 29 MHz from the central frequency. The details of the oscillating mode are illustrated in Figure 17 where the frequency span is reduced to 20 KHz around the central frequency. We can see that, the signal purity is nearly the same as that obtained without the filter, shown in Figure 17. This has been confirmed by measuring the phase noise of the oscillator with and without the filter. The obtained results are shown in Figure 18 where we have nearly the same phase noise. This insures that the added optical filter improves the frequency selectivity of the oscillator to allow single mode operation without degrading its spectral purity. For frequency tuning, the laser diode module is replaced by another one to allow for DC current control. The main parameters of this on the shelf laser diode are shown in Table 1. These parameters have been obtained using a specific laser diode parameter extraction technique explained in ****. Again, the all optical-RF filter is used in the feedback loop but with Lf ⫽ 5 m. The dc currant is changed gradually from 5.6 mA to 6.5 mA. At 5.7 mA an oscillation mode appeared at 790 MHz, the second oscillation frequency appears at 828 MHz corresponding to a dc current of 5.8 mA, with 38 MHz separation between the two modes, this is the
Figure 15 A magnified picture of the output spectrum of the oscillator showing its spectral purity without the filter
Figure 16 The output spectrum of the OEO after inserting the filter in the optical loop, L ⫽ 100 m
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Figure 18 The phase noise of the oscillator (semi-log scale) with and without the filter
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Figure 19 Oscillation modes for two different dc driving current Io ⫽ 5.7 mA and 5.8 mA
9. G. Keiser, Optical fiber communication, 3rd ed., McGraw-Hill, New York, 2000. 10. D. Strekalov, D. Aveline, N. Yu, R. Thompson, A.B. Matsko, and L. Maleki, Stabilizing an optoelectronic microwave oscillator with photonic filters, J Lightwave Technol 21 (2003), 3052-3061. 11. S. Tedjini, A. Ho-quoc, and D.A.M. Khalil, All-optical network as microwave and millimeter-wave circuits, IEEE Trans Microwave Theory Techniques 43 (1995), 2428-2434. 12. H.E. Kotb, A.E. Safwat, H. Boghdady, and D. Khalil, Tuning of an RF optoelectronic oscillator, Int Topical Meeting Microwave Photon 2006. MWP ’06, (2006), 1-4. 13. H.E. Kotb and D. Khalil, Multiple quantum well laser diode parameter extraction using the IM response, EUROCON, 2007, The International Conference on “Computer as a Tool”, (2007), 1256-1262. 14. R.S. Tucker, High-speed modulation of semiconductor lasers, J Lightwave Technol 3 (1985) 1180-1192. © 2008 Wiley Periodicals, Inc.
FSR between two consecutive modes of the optical filter. It must be noted that between these two current values, no output oscillation from the oscillator has been recorded. Therefore, the oscillator can be controlled digitally as oscillations exist at specific current values. The two oscillation points are shown in Figure 8. It is clear that they are near the tuning curve. Figure 19 shows the spectral measurement performed for two values for Io: 5.6 mA and 5.7 mA. The insertion of the optical filter in the feedback loop has resulted in higher order mode suppression of about 30 dB relative to the fundamental oscillating mode. 6. CONCLUSION
We have demonstrated the implementation of an RF OES at about 1.1 GHz using direct modulation of a semiconductor laser and an optical feedback with a single mode optical fiber. The demonstrated system has been based on existing laser diode and detectors modules. The system has also been implemented with low cost on the shelf components which results in a reduced cost RF OES. We have also demonstrated the effect of an all optical RF filter on the oscillator performance and showed that it can be used to transform the system to single frequency operation with more than 50 dB mode suppression ratio without affecting the signal purity (the associated phase noise). The oscillator tuning with the aid of the laser diode bias control is also demonstrated. This opens the door for a new class of RF OESs based on the all optical RF filters and directly modulated semiconductor laser diode which allows a significant reduction in the oscillator size and cost. REFERENCES 1. X.S. Yao and L. Maleki, Optoelectronic oscillator for photonic systems, IEEE J Quantum Electron 32 (1996), 1141-1149. 2. X.S. Yao and L. Maleki, A light-induced microwave oscillator, TDA Progress (1995), 47-68. 3. X.S. Yao and L. Maleki, A novel photonic oscillator, TDA Prog (1996), 32-43. 4. X.S. Yao and L. Maleki, Multiloop optoelectronic oscillator, IEEE J Quantum Electron 36 (2000), 79-84. 5. W. Zhou and G. Blasche, Injection-locked dual opto-electronic oscillator with ultra-low phase noise and ultra-low spurious level, IEEE Trans Microwave Theory Tech 53 (2005), 929-933. 6. D. Eliyahu and L. Maleki, Low phase noise and spurious level in multiloop optoelectronic oscillator, Proc IEEE Int Frequency Control Symp (2003), 405-410. 7. W. Zhou and G. Blasche, Injection-locked dual opto-electronic oscillator with ultra-low phase noise and ultra-low spurious level, IEEE Trans Microwave Theory Tech 53 (2005), 929-933. 8. M. Ming and K. Liu, Principles and applications of optical communications chapter 12, Irwin Professional Publishing, 1996.
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DUAL-BAND BANDPASS FILTER USING DEFECTED GROUND STRUCTURE Akhilesh Mohan and Animesh Biswas Department of Electrical Engineering, Indian Institute of Technology, Kanpur 208016, India; Corresponding author:
[email protected] Received 1 June 2008 ABSTRACT: Dual-band bandpass filters using defected ground structures (DGS) are presented in this letter. Dual-band characteristics are obtained using the two resonant frequencies (modes) of a novel DGS. Two configurations of dual-band bandpass filters are presented in this article. Both the filters are designed, fabricated, and measured. Measured results show good agreement with the simulated results. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 475– 479, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24067 Key words: microstrip filter; dual-band filter; defected ground structure 1. INTRODUCTION
Recent development in wireless communication systems has created a need of RF circuits with a dual-passband operation. The dual-band bandpass filters have been widely studied in the literature [1-4]. In [1], a dual band was implemented as a combination of two individual filters with two specific single passbands. By cascading a wideband passband filter and a stopband filter, a dual-band bandpass filter is achieved, but this result in large circuitry size [2]. Stepped impedance [3] and dual mode resonators [4] had also been used for dual-band operations. Dual-band bandpass filter using defected ground structure (DGS) has been proposed [5] but it uses the property of stepped impedance resonator and DGS resonator is added to improve its performance. In this letter, a novel DGS has been analyzed and it is used to design the dual-band bandpass filters. The coupled resonators are fed properly by the feeding lines to realize the dual passband. Two dual-band bandpass filters based on novel DGS resonators have been designed, fabricated, and tested. 2. A NOVEL DGS RESONATOR
A novel DGS has been shown in Figure 1. This novel DGS is realized by etching an equilateral triangular slot which is splitted along one of its arms. Because of the impedance discontinuity, the DGS behaves as a parallel resonator, which will form a band-gap
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