A TUNABLE LASER BASED ON NANO-OPTO-MECHANICAL SYSTEM M. Ren1, 2, H. Cai2, Y. D. Gu2, P. Kropelnicki2, A. B. Randles2 and A. Q. Liu1, 2†

1School of Electrical & Electronic Engineering, Nanyang Technological University, SINGAPORE 639798

2 Institute of Microelectronics, A*STAR, SINGAPORE 117685 ABSTRACT

This paper presents an external cavity tunable laser based on nano-opto-mechanical system by integrating the gain laser diode and the opto-mechanical ring resonators on a silicon chip. An optical force controlled tuning approach is demonstrated whereby the lasing light itself adjusts the lasing wavelength by controlling the mechanical displacement of the silicon ring resonator. In the experiments, a 24-nm wavelength tuning is realized due to a deflection of 14-nm. The optomechanical wavelength tuning coefficient is 214 GHz/nm. The demonstrated device has potential applications for optical communication system, pulse trapping/release, and chemical sensing, with easy on-chip integration on a silicon platform. INTRODUCTION

Tunable lasers are employed to reduce the amount of spare lasers with specific wavelengths for cost-effective wavelength division multiplexing systems or for passive optical network systems [1-2]. There are several approaches for wavelength tuning, such as by changing the real part of the effective refractive index, neff, or the length of the laser cavity, which are achievable by electronic control current or voltage. The refractive index control is available either by free-carrier plasma effect or by thermal tuning [3-4]. Both controls usually exhibit the essential disadvantage that the electrical power must be supplied, which heats up the devices and deteriorates other laser parameters such as optical power and cavity efficiency. Micro-electromechanical system (MEMS) technology has been used for external-cavity tunable lasers without power consumption due to the electrostatic force, but they suffer from low external cavity efficiency due to the free-space light travelling [5-7]. By applying nano-waveguide for light guiding in the external cavity may improve the laser performance with lower transmission loss. Nano-electromechanical system technology can be adopted to actuate nano-waveguide without electrical power consumption [8-9]. However, the nano-sized beam increases the impedance, making the devices susceptible to RF noise.

Optical force is a promising approach to actuate free-hanging nano-sized waveguide without electrical power consumption, which is compatible for high density on-chip integration [11-13]. In ring resonator based structures, refractive index can be changed by the movement of the free-standing ring resonator, thus making it possible to use optical force for wavelength tuning [14-15].

In this paper, a gain laser diode and opto-mechanical ring resonators are integrated to construct an external cavity tunable laser on a silicon chip. The lasing wavelength is

tuned by controlling the displacement of the free-standing ring resonator. Except for the pumping power applied on the gain chip, no additional electrical power is needed for the tuning of the effective refractive index. The driving force comes from the lasing light in the laser cavity itself.

DESIGN AND SIMULATION

Figure 1 illustrates the schematic of the optical force driven tunable laser. It is constructed by a commercial gain chip and a coupled ring structure (CRS). For the two ring resonators that couples with each other, one ring is fixed on the silicon dioxide (SiO2) layer, while the other ring is free-standing and can be actuated. The movable ring is supported by an anchor at the center and four nano-waveguide spokes. Both the bus-waveguide and the ring resonators are 450 nm in width and 220 nm in height. An air gap is formed between the free-standing ring resonator and SiO2 layer. The CRS functions as a wavelength-selective mirror: the broadband light provided by the gain chip is reflected by the CRS, and the reflectivity varies for different wavelengths. The wavelength with maximal reflectivity is called the gain peak wavelength, which obtains the highest gain while suppressing other modes, and is finally lased out from the laser cavity.

Lasing

Gain Diode

ActuatedFixed

Figure 1: Schematic of tunable laser based on nano-opto-mechanical system.

When the gain provided by the laser diode exceeds the loss in the entire cavity, it is lased at the gain peak wavelength λr. At this state, the lasing wavelength λr satisfies the resonant condition of both ring resonators. Therefore, the light constrained in the free-standing ring induces a gradient optical force, which deflects the free-standing waveguide with a nano-scale displacement d (Fig. 2(a)). Consequently, the nano displacement changes the effective refractive index of the ring resonator, and thus tunes the gain peak

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wavelength. The position of λr is a function of d. Nano-scale displacement usually causes nano-scale wavelength shift for a single ring resonator [10]. According to Vernier Effect, the wavelength tuning ability is amplified by tens fold for the coupled ring structure. In this paper, the optomechanical wavelength tuning coefficient is ∂λr / ∂d = 1.36·exp(0.03d), as indicated in Fig. 2(b).

dFopt

0 10 20 300

25

50

75

Lasi

ng W

avel

engt

h Tu

ning

Δλ r (

nm)

Displacement d (nm) (a) (b)

Figure 2: (a) Illustration of deflection under optical force; (b) calculation of the optomechanical lasing wavelength tuning coefficient of the coupled ring structure. The gradient optical force can be expressed as [12]

1

2 2 1 1 20

2,

(( ) 2 ) ( )e r

optr r r i e

PF

dcτ λ

λ λ λ π λ τ τ

−

− − −

∂= − ⋅∂− ⋅ + +

(1)

where P is the light power confined in the bus waveguide, λr is the lasing wavelength generated in the external cavity tunable laser, λ0 is the resonance determined by the coupled ring structure. 1/τe and 1/τi are the extrinsic and intrinsic decay rate respectively.

0 5 10 15 20 250

2

4

6

8 Fmech

Fopt 0.4 mW Fopt 1.2 mW Fopt 1.6 mW Fopt 2.4 mW

Forc

e (n

N)

Displacement d (nm) Figure 3: Optical force Fopt and mechanical force Fmech vs. displacement d, under different optical powers.

The deflection of the free-standing waveguide and the

lasing light perform interaction in the laser cavity. The defection can be controlled by the lasing optical power, thus can be controlled by the pumping level of the laser diode. The deflection, on the other side, changes the resonance of the coupled ring structure, and shifts the lasing wavelength. The stable lasing wavelength λr is achieved when a balance

between the lasing power and the displacement is built up. The position of the lasing wavelength is determined by the optical power in the laser cavity.

Figure 3 presents the static analysis of the free-standing waveguide, under different optical powers. The static displacement positions are determined with the graphical method. The mechanical force Fmech is proportional to the effective stiffness kmech, which is 0.24 N/m for the free-standing ring, while the optical force Fopt is a function of both displacement d and the optical power P. The cross points indicate the state whereby force equilibrium is achieved: 0.opt mechF k d+ = (2) At optical power level of P = 1.2 mW, there are 3 cross points, and the free-standing waveguide is stabled at point a. Therefore, the free-standing waveguide has a 2-nm displacement. This displacement increases gradually with the optical power until it reaches 4.8 nm (point b). By further increasing the optical power, there is only one cross point. For example, at P = 2.4 mW, the force-equilibrium condition is achieved at point c.

0.0 0.8 1.6 2.4 3.20

5

10

15

20 Stable Unstable

Dis

plac

emen

t d (n

m)

Optical Power in Cavity (mW) Figure 4: Nonlinear displacement due to different optical powers in the laser cavity.

The nonlinear curve in Fig. 4 shows the achievable

displacement of the free-standing waveguide as a function of optical power in the laser cavity. Fig. 4 combines all the force-equilibrium points when the optical power increases from 0 to 3.2 mW. The stable points are plotted in the solid line and the unstable points are plotted in the dashed line. Only stable points are achievable in experiments. The control of displacement is continuous when the optical power is tuned from 0 to 1.6 mW. Due to the existence of the unstable force-equilibrium points, the curve has a step at point b and the displacement jumps from 4.8 nm to 15.2 nm, which shows the discontinuity in lasing wavelength tuning. FABRICATION AND EXPERIMENTS

The opto-mechanical tunable laser is fabricated on an SOI wafer using nano-silicon photonics fabrication processes and MEMS packaging technology. For the fabrication of the photonic external cavity, the silicon waveguide is patterned

jump

a b

c

a b

c

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by reactive-ion-etching process and the movable structure is formed by controlling the time of the HF vapor etching. The commercial gain laser diode is bonded to the SOI chip by using Au/Sn solder. The misalignment between the waveguides of the laser diode and the photonic external cavity is controlled within 200 nm. Figure 5 is the SEM image of the free-standing ring, which has a radius of 25 µm. An internal disk with a radius of 8 µm works as an anchor. Four 600-nm width spokes connect both the anchor and the free-standing ring. The air gap between the free-standing waveguide and the substrate is controlled to be 150-nm.

Figure 5: SEM image of the free-standing ring with 25-µm radius supported by an anchor with 8-µm radius and four spokes with 600-nm width.

In the experiment, the transmission spectrum of the

couple ring structure is studied as shown in Fig. 6. A broadband light is coupled in and out of the bus waveguide via tapered optical fibers, and the transmission spectrum is detected by an optical spectrum analyzer. The broadband light source covers the range from 1500 to 1610 nm. The absorption dips in Fig. 6 correspond to those reflection peaks in the reflection spectrum. Dip A is the resonance of the free standing ring, while dip B is the resonance of the fixed ring. At the initial state, the merged resonance is observed at λ1 since the resonance of the two ring resonators match at λ1. The resonance split is observed at λ2 and λ3, and the split distance is 0.13 nm and 0.16 nm, respectively. Once the free-standing waveguide deflects, the dips of the free-standing ring (i.e. dip A) is red-shifted while the dips of the fixed ring (i.e. dip B) is kept static. Therefore, the merged resonance moves from λ1 to λ2 and λ3.

Figure.7 shows the single mode lasing outputs under different gain currents. When the laser diode works at 330 mA, a single mode laser wavelength at λ1 = 1535.8 nm is detected. Subsequently, λ2 = 1538.8 nm and λ3 = 1541.8 nm are lased at 365 mA and 400 mA, respectively. The spacing between two adjacent lasing wavelengths is 3.0 nm and the output power increases from -4dBm to 0 dBm. When the pumping current of the laser diode is increased to 410 mA, the lasing wavelength is tuned to λ4 = 1524.7 nm, with a large wavelength jump towards to the blue detuning direction, and the lasing power increases to 1.5 dBm.

1534 1536 1538 1540 1542-65

-60

-55

-50

-45 Initial Transmission

Pow

er (d

Bm

)

Wavelength (nm) Figure 6: Experiments results of the static transmission spectrum of the coupled ring structure. The resonance of the fixed ring and the free-standing ring merges at λ1.

1520 1530 1540 1550 1560-50

-40

-30

-20

-10

0

10 330 mA 365 mA 400 mA 410 mA

Pow

er (d

Bm

)

Wavelength (nm)

λ1 λ2 λ3λ4

Figure 7: Experimental results of a set of single mode output at different gain currents.

300 350 400 4501490

1510

1530

1550

1570 Period 1 Period 2

Lasi

ng W

avel

engt

h (n

m)

Gain Current (mA)

λ1λ2

λ3

λ4

λ4_0

jumpgain

Figure 8: Illustration of lasing wavelength tuning tendency. The wavelength blue-jump corresponds to the discontinuity of the displacement.

The blue jump of lasing wavelength is illustrated in Fig.

8. First, the displacement of the free-standing waveguide has a non-continuous jump at point b as shown in Fig. 4, indicating the discontinuity of the lasing wavelength with the

Anchor

Spoke

Free-standing Ring λ1 λ2 λ3

jump

A B

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increase of optical power. Second, due to the periodical property of the ring resonator, the coupled ring structure has two wavelength-selective periods within the wavelength range of 1490 to 1570 nm. On the other hand, the 3-dB gain spectrum of the laser diode convers 1510 to 1550 nm, which does not match with any of the two periods. As a result, the tuning of lasing wavelength is non-monotonic. It shifts from λ3 to λ4, instead of λ4_0, which is out of the gain spectrum. In the experiment, the acquired tuning range is 24 nm (3000 GHz), which corresponds to a 14-nm displacement of the free-standing waveguide. The optomechanical wavelength tuning coefficient is 214 GHz/nm. CONCLUSION

In conclusion, a tunable laser based on nano-opto-mechanical system is designed, fabricated and experimented. Optical power, instead of traditional electrical power, is applied for lasing wavelength tuning. A 24-nm tuning range is obtained with a mechanical displacement of 14 nm. The tuning process exhibits nonlinear mechanism due to the interaction between the lasing light and the mechanical movement. It has prospective applications in laser tuning, nonlinear signal processing and on-chip cell-manipulation.

ACKNOWLEDGEMENTS

This work was supported by the Science and Engineering Research Council of A*STAR (Agency for Science, Technology and Research), SERC Grant 1021650084.

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CONTACT

†A. Q. Liu, tel: +65-67904336; eaqliu@ntu.edu.sg.

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