698
-All-optical Flip-flop based on Optical Beating and Bistability in an
Injection-locked Fabry-Perot Laser Diode
Junsu Kim1, Hyuek Jae Lee2, and Chang-Soo Park1* 1
School of Electrical Engineering and Computer Science, Gwangju Institute of Science and Technology, Gwangju 61005, Korea
2
Department of Information & Communication Engineering Kyungnam University, Changwon 51767, Korea (Received November 16, 2016 : revised November 29, 2016 : accepted November 29, 2016)
We report a new all-optical flip-flop (AOFF) with a quite simple structure, using optical beating in an injection-locked Fabry-Perot laser diode (FP-LD) with optical bistability. While conventional AOFF methods using an injection-locked FP-LD require additional devices such as secondary FP-LDs or polarization controllers for reset operation, the proposed method can be implemented using only a single commercially available FP-LD with set and reset signals. The optical beating induces intensity fluctuations inside the FP-LD, and releases the locking state to the reset state. Even though we demonstrated the AOFF at 100 Mbit/s, we expect that its operation rate could extend to 10 Gbit/s, according to the limit of the FP-LD’s frequency response.
Keywords : All-optical flip-flop (AOFF), Injection locking, Optical bistability, Optical beating, Fabry-Perot laser diode (FP-LD)
OCIS codes : (060.1155) All-optical networks; (060.2330) Fiber optics communications; (230.1150) All-optical devices
*Corresponding author: csp@gist.ac.kr
Color versions of one or more of the figures in this paper are available online.
*
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/ licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
*Copyright 2016 Optical Society of Korea
I. INTRODUCTION
All-optical flip-flops (AOFFs) have been studied extensively for self-routing or buffering in optical packet switching systems [1, 2]. Optical bistable devices are generally used to develop AOFFs with set and reset signals. The set operation can easily be realized using an injection-locking method, but realizing the reset operation is more difficult in an all-optical context. This is because it is not easy to reduce the optical power of the locked signal to return it to the unlocked state, which is called the “negative” optical pulse for switching off. Solving this problem requires either combining electronic circuits on polarization-bistable laser [3], or a bistable device consisting of two Fabry-Perot laser diodes (FP-LDs) [4]. The polarization bistability depends on the transverse electric (TE) and transverse magnetic (TM) modes of the bistable device. Therefore, the applied bias current needs to be accurately controlled by the set and reset signals.
These techniques for the set and reset operations have recently been extended to such uses as architectures for a coupled ring laser, coupled Mach-Zehnder interferometers, and coupled nonlinear polarization switches [5-8]. In addition, an AOFF using two coupled, single-mode FP-LDs at high rate with high on-off contrast ratio has been reported [9]. However, these schemes are too complicated to realize large-scale optical buffer memories, due to their bulky structure. More recently, new AOFFs that can be constructed using only a single bistable device have been proposed [10, 11]. Still, these require a V-cavity laser with a complicated structure and polarization-dependent inputs.
In this paper, a novel scheme for an AOFF based on the bistability of an injection-locked FP-LD is proposed, and demonstrated experimentally. It achieves the reset operation via optical beating with the injected reset signal, without additional devices. The main advantages of the proposed AOFF are its simplicity and cost-effective structure, as it only uses
(a) (b)
(c)
FIG. 1. The process of the set operation, based on the bistability of an injection-locked FP-LD: (a) simple schematic for flip-flop operation, (b) hysteresis curve when AOFF is set, and (c) the optical spectrum in the injection-locked state.
(a) (b)
FIG. 2. The process of the reset operation, based on the bistability of an injection-locked FP-LD: (a) hysteresis curve when the AOFF is reset, and (b) the magnified optical spectrum around wavelength “4” in Fig. 1(c).
FIG. 3. The relationship between bistability, a CW holding beam, and a reset signal, to explain the reset operation. an ordinary, commercial FP-LD.
II. OPERATIONAL PRINCIPLE
The proposed AOFF is composed of a FP-LD and a 1×4 coupler, which has one output and three inputs for set, reset, and continuous-wave (CW) holding signals. The operation of the proposed AOFF is based on the bistability of the injection-locked FP-LD for its set operation, and the beating for its reset operation. Injection locking refers to a state in which the free-running longitudinal modes of the FP-LD are locked through direct coupling, or the injection of an external beam. The injection-locked FP-LD shows a single mode of operation at the wavelength of the external beam, with suppressed longitudinal modes. Moreover, injection locking can demonstrate bistability with two stable conditions, which depend on the injected power and wavelength detuning of the input beams. It is also noted that positive-wave-length detuning (redshifted wavepositive-wave-length) is more favorable for bistability characteristics [12].
The simple schematic of the proposed AOFF, and the process of the set operation based on the bistability of a FP-LD, are depicted in Figs. 1(a) and (b) respectively. Fig. 1(c) shows the optical spectrum of the FP-LD output before and after a set signal is applied. Initially, the intensity of the CW holding signal (ICW) is set so that injection locking
does not occur, which is position “a” in Fig. 1(b). The set signal (Iset) is applied at position “3” (i.e. at the right-hand
side of the third longitudinal mode), with positive wavelength detuning. This injected power increases the refractive index inside the bistable device, and the wavelengths of the longitudinal modes move to the longer wavelength side.
Finally, the third longitudinal mode near the CW holding signal is locked to the CW signal and total CW power is increased, with the power corresponding to position “b” of Fig. 1(b). This means that the AOFF has transitioned from the off to the on state. Even though the set signal has disappeared, injection locking is maintained due to the bistability, which is marked at position “c” of Fig. 1(b). Once injection locking occurs, it is difficult to unlock it without reducing the intensity of the CW holding beam for the reset operation.
Figs. 2 and 3 illustrate the process of the reset operation for the AOFF, as proposed in the paper. The region around wavelength “4” in Fig. 1 is magnified in Fig. 2, to provide a more detailed explanation. If the wavelength of a reset
FIG. 4. The measured bistability characteristic of an injection-locked FP-LD. Here λd is the wavelength difference (or wavelength detuning) between an external injected beam and a longitudinal mode of the FP-LD.
signal is sufficiently close to a CW holding signal, the efficient refractive index of the FP-LD changes in accordance with the beat frequency. In other words, the FP-LD acts as a sort of heterodyne detector for the two different signals with optical beating. We must pay special attention here to the region of optical beating (i.e. the reset region in Fig. 2), which exists around the locking wavelength of the CW holding beam. If a reset signal is applied, the locking condition changes from point “c” to “d,” as shown in Fig. 2(a). Without the reset signal, the locking condition goes back to the original point “a.”
To analyze the reset operation, let exp
exp and expexp be the
optical field of a CW holding signal and a reset signal, respectively. These two signals are combined, and a beat signal is generated inside the FP-LD. The absolute square of the sum of the complex amplitudes is given by
.
(1) The intensity is proportional to the absolute-square value of the complex amplitude; thus the total intensity can be described as
, (2) where and . The
intensities are depicted in Fig. 3, with the bistability relation-ship for the explanation of a reset operation. It is noted that the total intensity Itotal vibrates at ±2(ICWIreset)1/2 with
the beat frequency ωdiff/2π . Ilt and Iup respectively denote
the lower and upper thresholds of bistability. If Itotal is
decreased to less than the lower threshold Ilt (Itotal < Ilt), which
is point “R” in Fig. 3, the injection locking is immediately released. Nonetheless, three conditions must be satisfied for correct reset operation: First, the maximum (point “e” in Fig. 3) of Itotal should be less than the upper threshold
Iut; otherwise, AOFF becomes unstable, due to the repetition
of the reset and set operations. Second, the intensity of the CW holding signal ICW must be set between Ilt and Iut, and
be a little greater than the lower threshold Ilt for stable reset
operation, as per the first condition. Finally, the wavelength of the reset signal should not exactly equal that of the CW holding signal, which means ωreset ≠ ωCW. If the reset
signal has the same wavelength, Itotal becomes a constant
value, Itotal = ICW + Ireset + 2α (ICWIreset)1/2 where α = cos(ϕdiff),
and has the possibility of not meeting the reset condition (Itotal < Ilt).
III. EXPERIMENTAL RESULTS
Figure 4 shows the measured bistability characteristic of an injection-locked FP-LD. Here λd is the wavelength
difference (or wavelength detuning) between an external injected beam and the longitudinal mode of the FP-LD. Note that the wider the wavelength detuning, the higher the necessary injected power to the FP-LD, with stronger bistability. When λd ≥ 0.06 nm, the bistability characteristic
started growing wider. In the case of λd = 0.10 nm detuning,
the injected power for locking was greater than -4.2 dBm, and the bistability range was over 1.6 dB.
To verify the operation of the proposed AOFF, a set and a reset signal were applied to the FP-LD with λd =
0.10 nm detuning, where the wavelength of the CW hold beam was λcw = 1547.16 nm and that of the FP-LD mode
was λFP-LD = 1547.06 nm. Figure 5 shows the optical spectrum
for each operational step of the AOFF. Figure 5(a) shows the three longitudinal modes of the FP-LD (central mode 1547.06 nm). Figure 5(b) shows the spectrum of the unlocked FP-LD under the injection of a CW holding beam. Due to partial locking, the power of the left-hand-side mode was decreased, and that of the central mode was increased slightly. However, the power was insufficient to induce a set operation. The spectra for the set operation are shown in Figs. 5(c) and (d), with and without a set signal respectively. Even after the removal of the set signal, the set operation was maintained due to hysteresis. Figures 5(e) and (f) show the spectra of a reset operation. It was confirmed that once the reset condition was satisfied, the reset operation was maintained after the removal of the reset signal.
We scanned a tunable laser from 1547.04 to 1547.19 nm with several optical powers, to investigate the possible
(a) (b)
(c) (d)
(e) (f)
FIG. 5. The optical spectrum for each operational step of the AOFF when λd = 0.10 nm, (a) without any injection beam, (b) with the injection of a CW holding beam (OFF output), (c) with the injection of a CW holding beam and a set signal (set operation), (d) with only a CW holding beam (ON output), (e) with the injection of a CW holding beam and a reset signal (reset operation), and (f) with only a CW holding beam (OFF output).
FIG 6. Investigation of the set and reset wavelength regions.
FIG. 7. Experimental setup for testing the proposed AOFF. T-LD: Tunable Laser Diode; Mod: Optical Modulator; PC: Polarization Controller; EDFA: Er-doped Fiber Amplifier; OBF: Optical Bandpass Filter; OA: Optical Attenuator; OTDL: Optical Tunable Delay Line; FP-LD: Fabry-Perot Laser Diode; Amp: Electrical Amplifier.
wavelength ranges to achieve set and reset operations; the results are shown in Fig. 6. The available wavelength range could be increased with an increase in the set signal power (~0.11 nm at -10 dBm, compared to ~0.05 nm at -12 dBm); on the other hand, the reset region was maintained within a certain range (~0.03 nm) around the CW wavelength, irrespective of the reset signal power.
The experimental setup is shown in Fig. 7. Tunable lasers T-LD1 and T-LD2 were used for the set and the reset signals respectively. To generate the set and reset signals with the same data pattern (Non-Return-to-Zero (NRZ) signal at 100 Mbit/s), the data and data bar from the pulse pattern generator were applied to the Mach-Zehnder modulators (the input of Mod1 and the input bar of Mod2). The time delay between set and reset signals was adjusted by the optical tunable delay line (OTDL), and the FP-LD for the AOFF was biased at 13.4 mA. The measured Ilt and Iup
were -5.8 and -4.2 dBm for λd = 0.10 nm detuning, and
the CW holding beam (ICW) was set to -5.5 dBm at
1547.16 nm (λCW). Ireset was set to greater than -7.5 dBm
at 1547.15 nm (λreset) for the reset operation. Figures 8(a)
and (b) show the respective oscilloscope traces of the set (①), reset (②), and output (③) for time delays of 22 ns and 55 ns. Here CW denotes the injected locking output
by the CW holding beam. In addition, I ′CW, I ′set, and I ′seset
represent the outputs reflected from the FP-LD without locking. As shown in Figs. 8(a)-③ and (b)-③, the reflected reset signal I ′seset was not completely filtered out by OBF3
while resetting, due to very close wavelengths of Δλ = λCW
- λreset = 0.01 nm. The average on-off contrast ratio of the
(a)
(b)
I௪ᇱ +I௦௧ᇱ
I௪
I௪ᇱ
Set signal Reset signal Output
O p tic al Pow er (4 57 µ W /d iv ) I௪ᇱ +I௦௧ᇱ I௪ᇱ
Set signal Reset signal Output
Op tic al P o w er (4 57µ W /d iv )
Time (20.0 ns/div) Time (20.0 ns/div) Time (20.0 ns/div)
Time (20.0 ns/div) Time (20.0 ns/div) Time (20.0 ns/div) I௪
FIG. 8. Oscilloscope traces of the AOFF output, when the time delay between set and reset signals is (a) 22 ns and (b) 55 ns: set (①), reset (②), and output (③).
IV. CONCLUSION AND DISCUSSION
A new AOFF with a quite simple structure, based on optical beating in an injection-locked FP-LD with optical bistability, has been proposed and experimentally demonstrated. The AOFF can be constructed with only an FP-LD, which means that the method would be very advantageous for large-scale optical memory implementations. Even though the experiment was conducted at 100 Mbit/s, the operation speed may be extended to a few or even 10 Gbit/s, as predicted in other literature [13]. The low on-off contrast ratio and the crosstalk problem in resetting should be improved for practical use by maintaining a simple structure. One solution is to replace an FP-LD with a single-mode FP-LD, as in [9, 12]. As pointed out in the literature, a high on-off contrast ratio of over 35 dB can be obtained when the self-locked dominant mode of a single-mode FP-LD is set as the output. In addition, because the wavelength of the output can be far from that of a CW holding beam, the crosstalk problem is naturally avoided. An AOFF using a single-mode FP-LD will be reported in a future paper.
ACKNOWLEDGMENT
This material is based upon work supported by the Ministry of Trade, Industry and Energy (MOTIE, Korea)
under Industrial Technology Innovation Program. No. 10049151, “Development of Subnanometer Interferometer system.”
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