40-Gb/s directly-modulated photonic crystal lasers under optical injection-locking

40-Gb/s directly-modulated photonic crystal

lasers under optical injection-locking

Chin-Hui Chen,1,*

Koji Takeda,1 Akihiko Shinya,

2 Kengo Nozaki,

2 Tomonari Sato,

1

Yoshihiro Kawaguchi,1 Masaya Notomi,

2 and Shinji Matsuo

1

1NTT Photonics Laboratories, NTT Corp., 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan 2NTT Basic Research Laboratories, NTT Corp., 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan

*[email protected]

Abstract: CMOS integrated circuits (IC) usually requires high data

bandwidth for off-chip input/output (I/O) data transport with sufficiently

low power consumption in order to overcome pin-count limitation. In order

to meet future requirements of photonic network interconnect, we propose

an optical output device based on an optical injection-locked photonic

crystal (PhC) laser to realize low-power and high-speed off-chip

interconnects. This device enables ultralow-power operation and is suitable

for highly integrated photonic circuits because of its strong light-matter

interaction in the PhC nanocavity and ultra-compact size. High-speed

operation is achieved by using the optical injection-locking (OIL) technique,

which has been shown as an effective means to enhance modulation

bandwidth beyond the relaxation resonance frequency limit. In this paper,

we report experimental results of the OIL-PhC laser under various injection

conditions and also demonstrate 40-Gb/s large-signal direct modulation with

an ultralow energy consumption of 6.6 fJ/bit.

©2011 Optical Society of America

OCIS codes: (140.3520) Lasers, injection-locked; (200.4650) Optical interconnects; (230.5298)

Photonic crystals.

References and links

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#151793 - $15.00 USD Received 26 Jul 2011; revised 20 Aug 2011; accepted 21 Aug 2011; published 23 Aug 2011(C) 2011 OSA 29 August 2011 / Vol. 19, No. 18 / OPTICS EXPRESS 17669

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1. Introduction

According to International Technology Roadmap for Semiconductors (ITRS), in order to

realize optical interconnects that are competitive with the electrical interconnects in the 2020

– 2025 timeframe, the energy budget of optical devices should be less than 2 ~10 fJ/bit for on-

chip interconnects and 10 ~20 fJ/bit for off-chip interconnects [1]. While conventional

photonic devices are often large in physical dimensions and consume a large amount of

energy, photonic crystal (PhC) has been well recognized as a strong candidate for building

highly integrated photonic network-on-chips (NoCs) to overcome the scaling limits of CMOS

technology [2]. The ultra-compactness and low-power consumption because of strong light-

matter interaction (small mode volume and high Q-factor) in PhC nanocavities have therefore

created new opportunities for applications in power-efficient high-speed optical interconnects.

Figure 1 shows an example of photonic NoC configuration that consists of a photonic plane

overlaid on a CMOS electronic plane.

Fig. 1. Photonic NoC with a photonic plane integrated with a CMOS plane of a many-core

processor. Inset shows the proposed three-terminal output device.

Generally speaking, there are two schemes to convert electrical signals to optical signals in

a photonic NoC: (1) on-chip directly modulated lasers and (2) optical external modulators

with off-chip light sources. Compared to the latter case, the on-chip directly modulated laser

offers several advantages including compact size, high optical power efficiency, and low

energy consumption. The compact size can be realized not only by the wavelength-scale

nanocavity of lasers but also by eliminating the massively distributed waveguides in chip

packaging, which are frequently used in the scheme of off-chip light sources. Without the

additional loss due to fiber coupling and long-distance propagation in the distributed

waveguides, the optical power can therefore be used more efficiently with on-chip directly

modulated lasers. On the other hand, when we take the energy consumption of the transmitter

driver circuits into account, a well-designed driver circuit for on-chip directly modulated

lasers has comparable operating energy with the on-chip laser itself, while for the scheme of

using an external modulator, the overall energy consumption is usually dominated by the

driver circuit for the modulator, which is usually in the order of tens of pJ/bit [3]. In order to

meet the on-chip interconnect requirements in both energy and data rate, we have previously

demonstrated a high-speed ultra-compact directly modulated PhC laser with a low energy


consumption of 8.76 fJ/bit at 20-Gb/s [4]. In parallel to the effort towards on-chip

interconnects, we also explore the potentials of this directly modulated PhC laser to be used

for prospective off-chip interconnects.

The bandwidth requirement of optical output devices for off-chip interconnects is usually

more stringent than their on-chip counterparts, although the energy requirement can be

slightly relaxed. While we are pursuing higher chip performance with higher data rates, it

becomes more and more difficult for the off-chip I/O traffic to keep up with it. In order to

alleviate the pin-count limitations, optical output devices with high off-chip clock rate and

wavelength-division multiplexing (WDM) technology have emerged as promising means. In

practical use, however, the operational wavelength of optical devices may drift with

environment factors, which inevitably limits the minimal channel spacing in WDM. An

optical output device that can be operated at the high data rate with low energy consumption

is therefore highly desirable to reduce the total number of channels and to enable less

sophisticated transceiver designs with lower cost.

One of the major challenges for off-chip output devices is the direct modulation bandwidth

(BW) of a typical semiconductor laser, which is ultimately limited by its relaxation resonance

frequency (fr) and considered to be insufficient for future off-chip clock rate requirements.

Moreover, an adequate optical output power level of the output devices is required in order to

compensate for optical loss due to propagation and coupling to meet the requirements of

photodetector sensitivity at the receiver. While nanolasers are advantageous for low power

consumption, its sub-milliwatt (mW) output power may hinder its off-chip applications.

Optical injection-locking (OIL), on the other hand, is known to be an effective technique

to enhance both the fr and the modulation BW. The output power level and wavelength of an

injection-locked slave laser are mainly determined by the master laser at strong injection, and

therefore an output power at mW level can be easily obtained. Enhancement of fr to beyond

100 GHz has been demonstrated in VCSELs and DFB lasers [5]; however, reports on large-

signal modulation above 10 Gb/s are still missing. This is likely due to strong injection power

requirements together with practical limitations such as heating and gain compression at high

power level, which in turn limits the performance of large-signal modulation.

In this paper, we propose a three-terminal (input/output/injection) optical output device as

shown in the inset of Fig. 1. By utilizing a PhC laser with an external light injection operated

within the injection-locking range, we can realize low-power and high-speed off-chip

interconnects. Experimental results of small-signal modulation frequency response and optical

spectra under various injection conditions are reported. By optimizing the injection power and

wavelength detuning, we have successfully demonstrated flat broadband frequency response

and 40 Gb/s large-signal operation with an energy cost of 6.6 fJ/bit. The main focus of the

experiment was on modulation speed enhancement with a low energy cost operation.

Assuming the power dissipated by a properly designed electronic driver circuit for our on-

chip directly modulated laser is comparable to the power consumed by the laser itself as

suggested in [3], we can then estimate the total power of the transmitter with the proposed

BH-PhC laser to be less than 20 fJ/bit at 40Gbps. Compared with applying an off-chip

modulator that requires a high power (~25 pJ/bit) [3] wideband driver to deliver large voltage

swings, our directly modulated BH-PhC laser can provide a power efficient solution for next-

generation NoCs.

2. Buried heterostructure photonic crystal laser (BH-PhC laser)

The PhC laser contains an ultra-small buried heterostructure (BH) active region embedded in

a PhC air-bridge structure as shown in Fig. 2(a). Compared with a typical PhC configuration

formed in a thin membrane of gain material (usually InGaAsP), the introduction of the BH

region greatly improves the thermal conductivity as well as the confinements of both carriers

and photons in the cavity [2]. The BH active region with a size of 4 × 0.3 × 0.16 µm3 is placed

within a line-defect PhC waveguide (WG) in an InP slab and consists of three InGaAs


quantum wells with a 1.55 µm photoluminescence (PL) peak, which is sandwiched between

InGaAsP barrier layers with a 1.35 µm PL peak.

Due to the high thermal conductivity of the surrounding InP that is more than ten times

higher than that of InGaAsP, the generated heat can easily escape from the cavity. An index-

modulated mode-gap cavity with ultrahigh-Q and small mode volume [6] is formed by the BH

region in the line-defect PhC waveguide as a result of the higher refractive index of the active

region than the neighboring InP. The BH region also effectively confines the generated

carriers, and therefore a better overlap between carriers and photons provides efficient

pumping.

The line-defect PhC waveguide also serves as an input waveguide as shown in Fig. 2(b)

because InP is transparent to the 1.3 µm pumping light and thus undesired absorption outside

the cavity is ignorable. Figure 2(c) shows the FDTD-calculated mode profile without an

output waveguide. The output waveguide, as shown in Fig. 2(b), is placed at an offset position

with respect to the cavity, which enables optimized coupling between the cavity and itself [4].

This output waveguide is also used for external light injection for later experiments.

Fig. 2. (a) Scanning electron microscope (SEM) images of the top view and (b) cross-sectional

view of the fabricated BH-PhC laser. (c) The FDTD mode profile of the PhC cavity calculated

without output/injection waveguide. (d) Light-in-light-out curve (L-L) of the laser. A threshold

is observed at 11 µW pump power.

The light-in-light-out (L-L) curve of this laser is shown in Fig. 2(d). A threshold power of

the BH-PhC laser is observed at 11 µW incident pump power, and the maximal output power

is around 50 µW. It has been known [7] that fr of a semiconductor laser at free-running

depends on the number of photons in the cavity. And thus, as the number of photons increases

with the pump power for the laser, the fr continues to increase until practical factors such as

gain compression and heating take effects. Moreover, higher damping at higher frequencies

also limits the maximum BW. As a result, the maximal 3-dB BW is 15.7 GHz at a pump

power of 169 µW (not shown). All the power levels in this paper refer to the power within the

PhC waveguides, unless otherwise stated. All the measurements were carried out at room-

temperature.

3. BH-PhC laser + Optical injection-locking (OIL)

The frequency response of the BH-PhC laser under injection-locking was determined by

measuring the S21 parameter with a network analyzer (NA) using the apparatus shown in Fig.

3. The frequency response at free-running was also measured with the same apparatus except


for the light injection. The PhC laser was optically modulated by an electrical signal from port

1 of the NA via a LiNbO3 modulator (LN-Mod) to modulate the pump light. A CW light from

a tunable laser was coupled to the PhC laser cavity through the output/injection waveguide

with an optical circulator. The injected wavelength was detuned from the PhC lasing peak at

free-running by an amount of δ, and the injection power was adjusted by a digital variable

optical attenuator (VOA). The output of the PhC laser was taken after the circulator and a

90/10 coupler, and fed into an optical spectrum analyzer (OSA) and port 2 of the NA through

an erbium-doped fiber amplifier (EDFA) followed by an optical bandpass filter. An automated

computer program was used to precisely control the injection conditions, both the power level

and wavelength detuning value, and acquire data from the NA and OSA at each injection

condition.

Fig. 3. Schematic diagram of the apparatus used for the frequency response measurement.

VOA: variable optical attenuator. BPF: optical bandpass filter. LN-Mod: lithium-niobate

modulator. EDFA: erbium-doped fiber amplifier. OSA: optical spectrum analyzer.

Figures 4(a)–4(c) show the optical spectra and Figs. 4(d)–4(f) show the frequency

responses of the PhC laser biased at a pump power of 113 µW (~10 × Pth) with various

injection conditions as indicated on the figure (−15.07, −12.07, −9.07, and −6.07 dBm). When

the PhC laser is at free-running, the coupling between carriers and photons determines its fr.

The 3-dB BW of the PhC laser at a pump power of 113 µW is around 11 GHz as shown by the

black lines in all subplots of Fig. 4.

When the PhC laser is under light injection, the injected light boosts the stimulated

emission process and depletes more carriers (∆N < 0) in the PhC cavity, which results in a

reduction of the gain for the slave PhC laser. Even though the gain is lower than its threshold

value, externally stimulated emission from the master laser compensates for this reduction,

and thus the slave laser under injection-locking is lasing at the wavelength of the master laser

(λinj). Other than the main locked mode lasing at λinj, a shifted cavity mode (λcav) on its longer

wavelength side can also be easily seen in the optical spectrum. It is known that by analyzing

the nonlinear dynamics with the commonly used Lang-Kobayashi rate equations and solving

for the steady-state solutions, the amount of cavity shift can be obtained through the linewidth

enhancement factor (α): ∆λcav = - (λ02/2πc)·α·g·∆N/2, where g is the differential gain, ∆N is the

change of carrier density, λ0 is the wavelength of the master laser, and c is the speed of light

[8]. The cavity mode is red-shifted since ∆N is always a negative number.


Fig. 4. (a)–(c) Optical spectra and (d)–(f) frequency responses of OIL BH-PhC laser with

injection conditions indicated. Black lines in all figures represent the PhC laser at free-running

with a pump power of 113 µW. The injection wavelength (λinj) and the shifted cavity modes

(λcav) are also shown in the figures.

Figures 4(d)–4(f) show the corresponding frequency responses over the instrument-limited

20 GHz range. The frequency responses have been calibrated to the back-to-back

configuration, and also normalized to their DC values to find their 3-dB modulation BWs.

Apparent enhancement of the fr can be observed with higher injection power, while the

response is damped to a greater extent and a low-frequency roll-off becomes obvious for

higher injection power in each subfigure. The resonance frequency (fr’) at strong OIL is

dominated by the interaction between the photons from the injection light and from the shifted

cavity mode, and the frequency difference between these two modes matches to the fr’ shown

in the frequency responses [9]. A larger damping in the frequency response curves is found to

occur at larger δ for the same injection power. As δ increases, the shifted cavity mode is also


suppressed further on the optical spectra. With a subtle balance among fr’, damping factor,

and a low frequency roll-off [10,11], a broadband operation can be attained.

4. Direct modulation of OIL-PhC lasers at 40-Gb/s

Figure 5 shows the eye diagrams of directly modulated 40-Gb/s nonreturn-to-zero (NRZ)

signals (a) at the input, (b) at the output of the free-running PhC laser, and (c) at the output of

the injection-locked PhC laser. Since the maximal 3-dB BW of the free-running PhC laser was

only 15.7 GHz, it results in zero eye-opening when directly modulated at 40 Gb/s regardless

of the pump power. On the other hand, a clear 40-Gb/s eye opening under injection-locking

can be observed in Fig. 5(c) with an average pump power of 50 µW and an injection power of

163 µW at δ ~ + 0.1nm. The corresponding extinction ratio of the input eye was 4.5 dB, and

that of the output eye under injection-locking was 3.2 dB.

In the case of Fig. 5(c), the energy of the BH-PhC laser under injection-locking was

estimated to be 6.6 fJ at 40-Gb/s. Here a peak value of the pump power was used instead of

the average value. The total power consumption was then calculated as the sum of the pump

power (100 µW) and the injection power (163 µW). The energy efficiency of 6.6 fJ/bit is

considered to be small enough to meet the energy efficiency requirement in later years. It is

also the smallest number, to our best knowledge, among those ever reported for any type of

semiconductor lasers. We can see that the price of the additional injection power can be

regained by the enhanced data rate in terms of the energy cost per bit. The present

demonstration is focused on modulation speed enhancement with a low energy cost and is

limited by the available power in our setup. It is believed that further improvements of the

signal quality and modulation speed can be realized with higher pump power and injection

power.

The coupling loss in the pump side and in the output side of the device can be estimated to

be 10 dB and 8.5 dB, respectively. This results in a great rise of the required external power to

be around 53.8 fJ/bit in the current apparatus. However, in the future development we may

incorporate properly designed spot-size converters (loss < 1dB possible) [12,13] so that the

fiber coupling loss and required external power can be kept minimal, and the total power

consumption can still be within < 10 fJ/bit regime.

Fig. 5. Eye diagram for 40-Gb/s direct modulation. (a) Input signal. (b) PhC laser at free-

running with pump power of 50 µW. (c) OIL-PhC laser with pump power of 50 µW and

injection power of 163 µW in the PhC waveguide.

5. Conclusion

In this paper, we proposed and demonstrated a low-power and high-speed three-terminal

device based on an OIL-PhC laser for the next-generation photonic NoCs. Enhancement of

both fr and 3-dB BW of more than 20 GHz are presented with a flat broadband frequency

response by OIL. Large-signal direct modulation at 40 Gb/s have been successfully

demonstrated with an ultralow energy cost of 6.6fJ/bit, which is the smallest number ever

reported for semiconductor lasers. The OIL-PhC laser is therefore proven to be an effective

solution to meet the future needs of photonic NoCs especially for the stringent off-chip

requirements.


Acknowledgment

Part of this work was supported by the National Institute of Information and Communications

Technology (NICT), Japan.


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40-Gb/s directly-modulated photonic crystal lasers under optical injection-locking