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• Abstract

SECTION 1

## INTRODUCTION

Future optical networks will require high-speed routing with low power consumption and small footprint, which is becoming the bottleneck of data rate in the current networks. To realize the future optical networks, all-optical signal processing with photonic integrated circuits (PICs) has been expected to play an important role, because it will be able to remove optical-electrical-optical (OEO) conversion from the current networks [1], [2]. In particular, all-optical flip-flops (AOFFs) will be essential in the PICs, because they enable on-chip buffering, synchronizing, and regenerating functions. Many types of AOFFs have been proposed and demonstrated as the memory elements, such as coupled laser diodes (LDs) [3], [4], single or coupled Mach–Zehnder interferometers (MZIs) [5], [6], distributed feedback lasers with continuous-wave (CW) light injection [7], [8], polarization-bistable vertical-cavity surface-emitting lasers (VCSELs) [9], and so on.

Recently, we have proposed and demonstrated a novel type of AOFF based on multimode-interference (MMI) bistable LD (BLD) [10], [11]. We have also achieved single-mode lasing by applying distributed Bragg reflectors (DBRs) [12]. Since a DBR-LD does not need cleaved facets for its operation, multiple AOFFs can be monolithically integrated on a single PIC. Furthermore, because the switching mechanism of DBR-MMI-BLD does not rely on injection locking of lasing mode, it is relatively insensitive to the incoming signal wavelength. These properties are greatly advantageous in realizing cascaded operation of multiple AOFFs on chip.

In order to construct a large-scale all-optical logic circuit with reconfigurable design capability and maximum flexibility, the output wavelength tunability of DBR-MMI-BLD would be the next important issue. More specifically, sufficient tunability is required so that we can detune the lasing wavelength of adjacent AOFFs to ensure cascadability. In addition, large tunability is also attractive in filtering the output from individual AOFFs with shared access waveguide.

In this paper, we experimentally investigate the wavelength tunability and cascadability of the DBR-MMI-BLD AOFF. Lasing wavelength can be tuned with the range of 3.1 nm, which is sufficient to ensure the cascadability of multiple AOFFs. Maximum operable wavelength range of injected light is wider than 34 nm. In addition, dynamic flip-flop operation of the DBR-MMI-BLD is demonstrated with rising and falling time of faster than 280 and 228 ps. No waveform distortion is observed even when we tune the lasing wavelength.

SECTION 2

## OPERATION PRINCIPLE

The DBR-MMI-BLD is constructed from an active MMI coupler, two saturable absorbers (SAs), four DBRs, and access waveguides, as schematically shown in Fig. 1(a). Since the active MMI coupler is designed as a cross coupler, we can realize the two cross-coupled lasing states in the cavity. The two-mode bistability based on the cross-gain modulation (XGM) of the laser cavity provides the capability of an AOFF with the mode-holding function of SAs [10].

Fig. 1. (a) Schematic view of DBR-MMI-BLD AOFF. (b) SEM image of fabricated device.

We can control the refractive index of the DBR structures using the band-filling effect and free-carrier plasma effect of the injected current to tune the lasing wavelength. The Bragg wavelength $(\lambda_{B})$ is proportional to the refractive index, as TeX Source $$\lambda_{B} = 2n_{r}\Lambda\eqno{\hbox{(1)}}$$ where $n_{r}$ and $\Lambda$ are an effective refractive index of the waveguide and a grating pitch, respectively.

The current density $(J)$ is expressed as a function of carrier densities $(N)$: TeX Source $$J = ed(AN + BN^{2} + CN^{3})\eqno{\hbox{(2)}}$$ where $d$ is the thickness of the active layer; coefficients of $A$, $B$, and $C$ are the leakage constant, the band-to-band recombination constant, and the Auger recombination constant, respectively [13].

The refractive index change $(\Delta n)$ owing to the carrier can be assumed as proportional to the carrier density in the simplest case $(\Delta n_{r} \propto N)$ [14]. Thus the lasing wavelength shift is related to the injected current density using Eqs. (1) and (2).

SECTION 3

## DEVICE FABRICATION

The device was fabricated as illustrated in Fig. 2.

Fig. 2. (a) First MOVPE growth of InGaAsP core, MQW gain, and InP cap layers. (b) Active/passive etching and grating formation on InGaAsP core. (c) Second MOVPE growth of InP cladding, InP ridge, and InGaAs contact layers. (d) Ridge waveguide formation with wet-chemical etching. (e) Lift-off process of isolation layer. (f) Electrode formation. The figures are focused on the point of active/passive interface with grating structure.

A 300-nm unintentionally (u-) doped InGaAsP waveguide core, and InGaAsP multiple quantum wells (MQWs) were grown by metal-organic vapor phase epitaxy (MOVPE) on an n-InP substrate, separated by a 10-nm InP etch stop layer to form an offset quantum-well structure [15]. The gain peak of the waveguide core and MQWs were 1250 and 1540 nm, respectively. We applied 0.4% crystal strain only to the well layers of the MQWs to obtain higher gain for the transverse-electric (TE) mode, whereas no strain was applied to the other layers. After patterning to preserve the MQWs at the MMI and the SA regions, the MQWs at the passive regions were removed by wet-chemical etching. We deposited $\hbox{SiO}_{2}$ layer with thickness of 50 nm as masks for etching gratings. The first order gratings were patterned by electron-beam (EB) lithography, and the pattern was transferred to the $\hbox{SiO}_{2}$ masks by inductively coupled plasma (ICP) dry etching. The InGaAsP core layer was slightly etched using the bromine based solution [16]. Then, the $\hbox{SiO}_{2}$ mask and the InP cap were removed. After the cleaning of the sample, u/p-InP upper cladding layers, 850-nm p-InP ridge, and 200-nm p-InGaAs contact layers were regrown using the MOVPE. Consequently, we obtained the active/passive integrated structure with two-step MOVPE process.

After the regrowth, the p-InGaAs and the p-InP ridge layers were wet-chemically etched after the waveguide patterning, followed by a lift-off process of $\hbox{Al}_{2}\hbox{O}_{3}$ isolation layer. The InGaAs layer at the passive regions was etched out. The Ti/Au electrode was deposited using an EB evaporator, followed by the lift-off process.

A scanning electron microscope (SEM) image of the fabricated device is shown in Fig. 1(b). We designed the widths of waveguides, MMI, and the length of the MMI as 2, 12, and 650 $\mu\hbox{m}$, respectively. The cavity length was 1.7 mm, and the entire device length was 2.5 mm.

SECTION 4

## MEASUREMENT SETUP

The fabricated device was fixed on a Peltier cooler. Since it was needed to inject larger current at the wavelength-tuned condition as described below, we kept the temperature at 10 °C during the measurement.

A measurement setup is shown in Fig. 3. A tunable LD (TLD) emitted CW light with wavelength range of from 1530 to 1578 nm. The CW light was modulated into 10-ns-wide pulse shape with repetition period of 320 ns using a lithium niobate (LN) MZI modulator. The LN modulator was driven by a 10-GHz pulse pattern generator (PPG). The LN modulator could be bypassed using an optical mechanical switch when we evaluated static characteristics of the device. The light was amplified with an erbium-doped fiber amplifier (EDFA), followed by a band pass filter (BPF) with bandwidth of 1 nm to eliminate an amplified spontaneous emission (ASE) noise from the EDFA.

Fig. 3. Measurement setup of the AOFF. Mechanical optical switches were applied to change the injected light from CW to the pulses with the LN modulator.

The light was divided into two paths to act as the set and reset light. A fiber delay line (FDL) was inserted into the reset path, whose length corresponded to a 170-ns delay. Optical powers were controlled by variable optical attenuators (VOAs), followed by the polarization controllers (PCs) to control the polarization state to the TE mode. The set and reset light were injected into the device through circulators and lensed fibers.

The output light from the device was extracted through the circulators, and monitored with an optical spectrum analyzer (OSA), an optical power meter (OPM), and a digital sampling oscilloscope (DSO).

SECTION 5

## STATIC FLIP-FLOP SWITCHING

First we bypassed the LN modulator, injected bias current to the MMI, and monitored the output light. The DBR-MMI-BLD shows bistability between the on and off states owing to nonlinear absorption at the SAs. Threshold current and hysteresis width are 206 and 28 mA, respectively. Single-mode lasing is obtained, and the lasing spectrum is shown as the thickest blue plot in Fig. 4(a). The lasing wavelength, extinction ratio (ER), and side-mode suppression ratio (SMSR) are 1544.1 nm, 32.2 dB, and 27.7 dB, respectively. No current is injected to the DBRs in this initial case. We refer this condition as “Case 1” in this paper.

Fig. 4. (a) Superimposed lasing spectra of the DBR-MMI-BLD. The thickest blue plot shows the lasing spectrum without tuning current to the DBRs. Seven lasing modes were obtained with tuning current to the DBRs. (b) Wavelength detuning as a function of injected current densities. Red square plot and blue circle plot show the current densities injected into DBR(1R) and DBR(1L) as shown in Fig. 1(a). Black solid line shows the fitted curve of the plots with Eqs. (1) and (2).

When we inject current to the DBRs as shown in Fig. 3, the lasing wavelength is blue shifted and the maximum shift range is 3.1 nm in this experiment, which is consistent to the previous report of the conventional DBR laser [17]. Their lasing spectra are shown in Fig. 4(a) as superimposed plots. We can determine that this wavelength shift is due to the band-filling effect and the free-carrier plasma effect, because the other major effect for the refractive index, heat generation due to the current injection, causes a red shift.

Wavelength detunings are plotted in Fig. 4(b) as a function of the injected current densities to the DBRs. Required current density is approximately 2.7 $\hbox{kA/cm}^{2}$ in case of 3.1-nm wavelength shift. We refer this maximum blue-shifted condition as “Case 2” in this paper. The plot can be well fitted by the Eqs. (1) and (2) with parameters of ${\rm A} = 1.5 \times 10^{9}\ 1/\hbox{s}$, ${\rm B} = 8.0 \times 10^{-17}\ \hbox{m}^{3}/\hbox{s}$, and ${\rm C} = 5.0 \times 10^{-42}\ \hbox{m}^{6}/\hbox{s}$. We apply the proportionality constant of $3.5 \times 10^{-28}\ \hbox{m}^{3}$ between the refractive index change and carrier density. The SMSR and ER are higher than 24.7 and 21.5 dB, respectively, at the entire tuning range.

Next we injected the CW light with wavelength of 1550 nm to the device to obtain static flip-flop switching, and measured output power from the device at the lasing wavelength. The bias current to the MMI was set to 183 mA and 243 mA in the Case 1 and Case 2, respectively. The results are shown for Case 1 and Case 2 in Fig. 5(a) and (b), respectively. The set and reset light were injected from left-lower and right-lower access waveguide, respectively. The set and reset operation are plotted as red circle and blue square plots, respectively. In the Case 1, the flip-flop is set with injected light power higher than −12.1 dBm, and reset with injected power higher than −4.8 dBm, respectively. In a similar manner, threshold set and reset power of −13.1 dBm and −1.8 dBm are obtained in the Case 2, respectively. The output power from the device is lowered due to the free-carrier absorption at the DBRs in the latter case. The lasing light is suppressed when strong light is injected because of the XGM effect. Similar flip-flop switching behaviors are demonstrated in all other lasing conditions shown in Fig. 4(a). While the output power from the device was lower than the required switching power, it was mainly due to the coupling loss between the waveguides and fibers (5.7 dB/facet). Since the photonic integration will enhance the power efficiency of 11.4 dB, the cascading of flip-flops will be realized with semiconductor optical amplifiers, which can be integrated without any additional processes.

Fig. 5. Static flip-flop switching at the lasing wavelength of (a) 1544.1 nm and (b) 1541.0 nm with injected light wavelength of 1550 nm. Red circle plots and blue square plots correspond to set and reset operation, respectively. Switching light was injected through left lower port and right lower port ad the set and reset operation, respectively.

We measured threshold set and reset power as a function of wavelength of injected light, and results are shown in Fig. 6. The threshold set power and reset powers are plotted as red circle and blue square plots. The lasing wavelength of the Case 1 and Case 2 are noted as the black solid and dashed line, respectively. In the Case 1, the maximum operable range is 48 nm (from 1530 to 1578 nm). In a similar manner, the operable wavelength range is 34 nm (from 1530 to 1564 nm) in the Case 2. The laser bias currents are selected within a range of 188 ± 14 mA and 237 ± 13 mA for Case 1 and Case 2, respectively.

Fig. 6. Wavelength dependence of the required switching power and the bias current with lasing wavelength of (a) 1544.1 nm and (b) 1541.0 nm. Red circle plots and blue square plot show the required set power and reset power, respectively. Maximum operable bandwidths were 48 nm (1530–1578 nm) and 34 nm (1530–1564 nm) in the Case 1 and Case 2, respectively. The lasing wavelengths of two cases are noted as solid line (1544.1 nm) and dashed line (1541.0 nm).

The threshold power has smooth wavelength dependence if the wavelength of the injected light is far from the lasing wavelength. When the wavelength of the injected light is close to the lasing wavelength, it becomes difficult to reset the flip-flop. The flip-flop operation was not available when the wavelength of the injected light was within the range from 1543.3 nm to 1544.5 nm.

It is important that the flip-flop in the Case 1 can be switched by wavelength of 1541.0 nm as shown with dashed line in Fig. 6(a), which is the lasing wavelength of the Case 2. It means that one flip-flop can be controlled by the other flip-flops when the multiple devices are monolithically integrated on a single PIC. Although the resetting operation could not be obtained at 1544.1 nm in Case 2, this is because we could not inject current into other DBRs in the resetting port due to the experimental setup.

From these results, we can conclude that the DBR-MMI-BLD is lasing in the single mode, and have wide operable wavelength range. Furthermore, the device can be controlled by the other flip-flops with wavelength tuning.

SECTION 6

## DYNAMIC FLIP-FLOP OPERATION

We measured dynamic responses of the DBR-MMI-BLD using the LN modulator and the DSO as shown in Fig. 3. Waveforms of dynamic AOFF operation are shown in Fig. 7. From the top to the bottom, each waveforms show the 10-ns-wide set pulses, reset pulses, output light from the device when the lasing wavelengths were set to the Case 1 $(\lambda = 1544.1\ \hbox{nm})$ and the Case 2 $(\lambda = 1541.0\ \hbox{nm})$. Wavelengths of the injected pulses were fixed to 1550.0 nm in this experiment.

Fig. 7. Dynamic flip-flop operation of the DBR-MMI-BLD. Each plots show (a) the set pulses, (b) the reset pulses, output light from the device of (c) Case 1 and (d) Case 2, respectively. The energies of set and reset pulses are (c) 4.08 and 44.1 pJ in Case 1, (d) 37.5 and 101 pJ in Case 2, respectively. The wavelengths of injected pulses are 1550.0 nm. Enlarged waveforms of (c) and (d) are shown in (e) and (f), respectively. The frequency of relaxation oscillation at the rising points is 0.9 GHz. The rising time corresponds to the relaxation frequency is 280 ps. Falling times are (e) 228 and (f) 146 ps.

Clear flip-flop operations are observed with both conditions, as shown in Fig. 7(c) and (d). The bias currents were 173 and 238 mA in the Case 1 and Case 2, respectively. The device starts lasing after the falling edge of the set pulses, and stops lasing at the rising edge of the reset pulses. The energies of the set and reset pulses are 4.08 and 44.1 pJ in the Case 1, respectively. Similarly, the energies are 37.5 and 101 pJ in the Case 2, respectively. The ERs at this dynamic measurement are 10.3 and 8.5 dB with the Case 1 and the Case 2, respectively. The ERs are diminished compared with the static measurement because of the ASE noise from the EDFA before the DSO.

The enlarged waveforms at the rising and falling edges are shown in Fig. 7(e) and (f). A relaxation oscillation is observed at the rising edges, with the frequency of 0.9 GHz for both cases. The rise time is 280 ps, which corresponds to the relaxation frequency of the cavity. There is no relaxation oscillation at the falling edges. The fall time is 228 ps and 146 ps for respective cases. We observe that the falling time is decreasing by introducing carrier injection to the DBRs because of the diminished photon life time due to the free-carrier absorption at the DBRs. On the other hand, the relaxation oscillation is not changed in both cases, because the decrease of steady-state photon density cancels the effect of diminished photon lifetime.

The rise and fall times are limited by relaxation frequency of the cavity and the photon lifetime, respectively, in this device. We can enhance these parameters by shrinking the device size.

SECTION 7

## CONCLUSION

We have demonstrated the AOFF with abilities of large operable bandwidth and wavelength tunability based on the MMI-BLD structure with the DBRs. The operable bandwidth was 48 nm at the initial condition. The tuning range of 3.1 nm will enable flexible and reconfigurable design of the future PICs. Furthermore, the dynamic set-reset flip-flop operation was successfully demonstrated with optical pulse injection without any waveform distortion even when we tuned the lasing wavelength. The rising and falling time were faster than 280 and 228 ps, respectively. Such functions of all-optical buffering with wavelength tunability and large operable bandwidth are highly attractive in the future PICs.

## Footnotes

This work was supported by Grant-in-Aid for Scientific Research (S) #20226008 and Grant-in-Aid for JSPS fellows, Japan Society for the Promotion of Science. The photomasks were made using the 8-inches EB writer F5112+VD01 (donated by ADVANTEST Corporation) of the University of Tokyo VLSI Design and Education Center (VDEC). Corresponding author: K. Takeda (e-mail: takeda@hotaka.t.u-tokyo.ac.jp).

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