Abstract: We measure stimulated Raman gain at 1550 nm in an ultra-small SOI strip waveguide, cross-section of 0.098 µm2. We obtain signalamplification of up to 0.7 dB in the counter-propagating configurationfor a sample length of 4.2 mm and using a diode pump at 1435 nm withpowers of <30 mW. The Raman amplifier has a figure-of-merit (FOM)of 57.47 dB/cm/W. This work shows the feasibility of ultrasmall SOIwaveguides for the development of SOI-based on-chip optical amplifiersand active photonic integrated circuits.
References and links1. J. S. Foresi, D. R. Lim, L. Liao, A. M. Agarwal, and L. C. Kimerling, “Small radius bends and large angle
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switch,” IEEE Phot. Tech. Lett. (to be published).5. S. Coffa, G. Franzo, and F. Priolo, “Light emission from Er-doped Si: materials properties, mechanisms, and
device performance,” MRS Bulletin, 23, 25–32 (1998)6. R. Claps, D. Dimitropoulos, Y. Han, and B. Jalali, “Observation of Raman emission in silicon waveguides at
7. R. Claps, D. Dimitropoulos, V. Raghunathan, Y. Han, and B. Jalali, “Observation of stimulated Raman amplifi-cation in silicon waveguides,” Opt. Express 11, 1731–1739 (2003),http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-15-1731
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(C) 2004 OSA 9 August 2004 / Vol. 12, No. 16 / OPTICS EXPRESS 3713#4687 - $15.00 US Received 29 June 2004; Revised 20 July 2004; accepted 20 July 2004
13. R. Claps, V. Raghunathan, D. Dimitropoulos, and B. Jalali, “Influence of nonlinear absorption on Raman ampli-fication in Silicon waveguides,” Opt. Express 12, 2774–2780 (2004),http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-12-2774
1. Introduction
Recently silicon-on-insulator (SOI) has emerged as an attractive materials system for photonicintegrated circuits (PICs). SOI offers the advantages for potentially being integrated with stan-dard silicon electronics. In addition, because of its compatibility with silicon planar processing,CMOS fabrication tools and techniques can be utilized. Furthermore, the large refractive-indexcontrast of SOI allows very tight modal confinement, which leads to device miniaturization.These advantages have already led to several reports of ultrasmall passive optical structuressuch as 90◦ bends, and T- or Y-branches with excellent performance [1]. For example, sharpbend radii of the order of 1 µm with low losses have been fabricated [2]; this particular capa-bility shows clearly the advantages of SOI for deeply scaled down PICs.
One important challenge in realizing greater use of this materials technology is to makea set of active devices based on SOI. Since silicon is centrosymmetric, it does not exhibit alinear electro-optic effect. However, in the last two years high-performance optical modula-tors and switches based on thermooptic and free-carrier dispersion effects have been demon-strated [3, 4]. Achieving on-chip light emission is an even more important capability and hasbeen a major component of research on active devices. For example, erbium doping has beencarefully explored but the results have been, thus far, limited [5]. Recently Claps et al. haveproposed and demonstrated the use of the Raman effect in silicon to achieve an on-chip am-plifier [6]. The large Raman gain coefficient of silicon (∼10 4 larger than that of silica) can, inprinciple, be used to achieve practical levels of gain with a diode laser pump. In this connection,Claps et al. have demonstrated spontaneous Raman emission [6], Raman amplification [7], andcoherent anti-Stokes Raman scattering (CARS) [8]. In these studies, they used a rib waveguidestructure that yielded a modal area of 5.4 µm2, from which signal amplification of 0.25 dB wasobserved with 1.6 W of coupled pump power [7].
Because the Raman effect is a nonlinear optical process, tighter optical confinement can leadto an increase of the efficiency of the process. Hence, from the viewpoints of practical SOI-device integration, further reduction in the transverse waveguide dimensions is a necessity.For this reduction to be realized, however, two experimental issues must be solved. First, thesidewall roughness of the waveguide must be lowered to reduce the high propagation loss.Second, the input and output coupling from fiber to waveguide must be efficient.
In this paper, we employ low-loss, ultrasmall-core SOI waveguides to demonstrate stimulatedRaman amplification at 1550 nm using a 1435 nm diode pump. We observe On-Off gains ofup to 0.7 dB for small pump powers of <30 mW. Our experiments make use of SOI stripwaveguide devices with a cross-section of 0.098 µm2.
2. Fabrication and experimental setup
The devices were patterned on 200 mm SOI Unibond wafers (SOITEC) with a 220 nm-thick,lightly p-doped silicon top layer on a 1 µm SiO2 layer. A 50 nm-thick oxide was depositedvia low pressure chemical vapor deposition (LPCVD) as a hard mask for the etching process.The patterns were defined by electron beam lithography using a Leica VB6-HR commercial100 keV system. The exposed wafers were then etched in a standard 200 mm CMOS line atIBM Watson Research Center [9]. The resist pattern was transferred to the oxide mask using aCF4/CHF3/Ar etch chemistry. After resist removal, the oxide mask was transferred to the topsilicon layer with an HBr-based etch. A second lithography step defined the polymer (n∼1.58)
(C) 2004 OSA 9 August 2004 / Vol. 12, No. 16 / OPTICS EXPRESS 3714#4687 - $15.00 US Received 29 June 2004; Revised 20 July 2004; accepted 20 July 2004
Fig. 1. Optical and SEM micrographs of fabricated SOI devices. (a) Waveguides of varyinglengths. (b) Zoom of polymer taper spot-size converter. (c) SEM of polymer with invertedsilicon taper tip
used, in conjunction, with an inverted waveguide taper as the spot-size converter-couplers. Thesamples were then cleaved on each side to enable edge-coupling. Figure 1 shows the opticaland scanning electron microscopy (SEM) micrographs of the fabricated devices. The sidewallangles were ∼90◦ and the roughness values were 5 nm. The final devices were 4.6 mm long.The polymer spot-size converter-couplers were 3 µm wide, 2 µm thick, and 200 µm long, witha tapered-tip size of 75 nm. The single-mode strip waveguides were 445 nm wide, 220 nmthick, 4.2 mm long, and support only the TE polarization.
The schematic of our experimental setup for measuring Raman gain is shown in Fig. 2. ACorning Lasertron diode, operating at 1435 nm, was used as our pump and a JDS Fitel broad-band noise source with power ∼1 mW, bandwidth ∼40 nm and centered at λ =1550 nm wasour signal source. The counter-propagating configuration for the pump and signal beams wasused to minimize other nonlinear optical processes, e.g., Four-Wave Mixing (FWM), that maybe phase-matched along the forward direction. The pump beam was sent to a pump-signal com-biner and was in-coupled into the input facet of the waveguide through a tapered polarization-maintaining (PM) fiber with a spot size of ∼2.5 µm, as depicted by Fig. 2. The pump beamwas then out-coupled into a receiving tapered fiber and demultiplexed into the pump chan-nel of a similar pump-signal combiner. Conversely, our broadband signal was coupled into thewaveguide via the combiner and the tapered fiber in the opposite direction. The power of thebroadband radiation was several orders of magnitude larger than the power of the spontaneousemission, centered at λ =1550.7 nm, generated in both forward and backward directions by thepump beam. Finally the counter-propagating broadband signal was coupled to the signal line of
(C) 2004 OSA 9 August 2004 / Vol. 12, No. 16 / OPTICS EXPRESS 3715#4687 - $15.00 US Received 29 June 2004; Revised 20 July 2004; accepted 20 July 2004
Fig. 2. Experimental setup for measuring stimulated Raman gain in SOI waveguides.
the input pump-signal combiner and subsequently monitored by an optical spectrum analyzer(OSA) with a 2 nm resolution. We used a high-bandwidth OSA detection setting in order toimprove the signal-to-noise ratio (SNR) of our device. However, in order to obtain the correctlinewidth of the gain spectrum, we used a higher-resolution, lower-bandwidth setting of 0.5 nm,as discussed below.
3. Results and discussions
The propagation loss of our waveguides was measured using the cutback method and found tobe 3.6±0.1 dB/cm at 1550 nm [2]. Input and output coupling losses were ∼1.5-2 dB/coupler.All results were for the TE polarization.
We measured the On-Off gain of the device, defined as, G = 10logR, where R is the outputpower while the pump is on divided by the output power while the pump is off. Figure 3(a)shows the measured Raman gain spectrum of the ultrasmall SOI waveguides. The input pumppower was 20.5 mW with an On-Off gain of 0.4 dB. The data exhibits a gain maximum atλ =1550.7 nm, which corresponds to the predicted ∆ν=15.6 THz (521 cm −1) Raman shift in sil-icon [6]. We measured accurately the Raman linewidth of ∆λ∼1 nm using the higher-resolutionOSA setting; this value agrees with the convolution of the pump beam linewidth and the siliconRaman linewidth. We also measured the spontaneous Raman spectrum, i.e., the pump in theabsence of the signal, for the same pump power using the same experimental parameters; thisis shown in Fig. 3(b) for comparison. Clearly, the stimulated Raman data agree well with thespontaneous Raman peak position and linewidth.
In order to examine the power dependence of the gain, we measured the On-Off gainversus the input pump power (i.e., power entering the waveguide) as shown in Fig. 4. Themaximum gain was G∼0.7 dB (15%) for a pump power of PR∼29 mW and for a waveg-uide length of L=4.2 mm. The Raman amplifier figure-of-merit (FOM), which we define asFOM = G/(PRL)=57.47 dB/cm/W, is approximately 3 orders of magnitude greater than previ-ously reported for SOI-based Raman amplifiers [7]; this increase in FOM shows that the gainscales approximately linearly with modal area. The data in Fig. 4 is approximately linear with aslope of 0.029 dB/mW. We compared this data to a numerical solution of the stimulated Ramandifferential equations [10], viz.
(C) 2004 OSA 9 August 2004 / Vol. 12, No. 16 / OPTICS EXPRESS 3716#4687 - $15.00 US Received 29 June 2004; Revised 20 July 2004; accepted 20 July 2004
Wavelength (nm) (a)1548 1549 1550 1551 1552 1553
On-
Off
Gai
n (d
B)
0.0
0.1
0.2
0.3
0.4Pump=20.5mW
Wavelength (nm) (b)1548 1549 1550 1551 1552 1553
Ram
an P
ower
(µW
)
0.0
0.1
0.2 Pump=20.5mW
Fig. 3. Stimulated (a) and spontaneous (b) Raman emission spectra with high-bandwidthOSA detection for SOI waveguides.
dPP(z)dz
= −νP
νRgRPP(z)PR(z)−αPP(z)−
(β
Aeff
)PP(z)2 (1)
dPR(z)dz
= αPR(z)−(
gR −βAeff
)PR(z)PP(z) (2)
where PP is the pump power, PR is the Raman power, νP and νR are the pump and signalfrequencies, α=3.6 dB/cm is the propagation loss of the waveguide, β =0.44 cm/GW is theTwo Photon Absorption (TPA) coefficient [7], A eff=0.059 µm2 is the effective modal area, andgR=29 cm/GW is the stimulated Raman scattering (SRS) coefficient. The solution takes intoaccount the effective pump power due to the finite pump bandwidth, i.e. Peff = PP/(1+ ∆νP
∆νR,Si),
where ∆νP=160 GHz and ∆νR=105 GHz [7, 11]. The effect of pump depletion, i.e., the couplingterm in equation (1), is also accounted for in the calculation and found to be negligible. Thecalculated data, shown as a dashed line in Fig. 4, has a slope that matches the experimental datato within ±10%. From this comparison, we estimate the SRS coefficient to be ∼29±4 cm/GW.
Our On-Off gain and SRS coefficient agree well with the results of Ref. 7, but there isclearly a discrepancy between the calculated and the experimental data since there is an off-set of ∼0.2 dB in the lower-power extrapolation of the linear gain. We believe this offset isattributed to a thermally-induced change of the tapered fiber tip at higher pump powers; this
(C) 2004 OSA 9 August 2004 / Vol. 12, No. 16 / OPTICS EXPRESS 3717#4687 - $15.00 US Received 29 June 2004; Revised 20 July 2004; accepted 20 July 2004
Pump Power (mW)0 10 20 30
On-
Off
Gai
n (d
B)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7 linear fit, m=0.029
calculated, m=0.029
Fig. 4. On-Off gain versus input pump power. The maximum gain is 0.7 dB (17%) with apump power of ∼29 mW. A linear fit with a slope of 0.029 dB/mW corresponds to an SRScoefficient, gR∼29 cm/GW.
was a reproducible effect, which caused fiber misalignment. It is possible to limit this effectby adding a temperature bias on the system and using piezoelectric XYZ actuators for moreaccurate and stable positioning. At present, our maximum gain is limited by the available pumppower. Either a higher-power pump diode laser or lower losses within the optical componentsof our experimental setup, e.g., beam combiners, connectors, and tapered fibers, would increasegain. Because of the tight confinement of our waveguides, other nonlinear processes, such asTPA and Stimulated Brillouin Scattering, can be present as well [7]. However, our calculationsindicate that these effects are negligible because of the low pump powers used. The effect ofTPA-induced free-carrier absorption was recently proposed as a limitation on the achievable Ra-man gain in SOI [12]. However, this effect is negligible in our experiments because the linearityof the power dependence on the spontaneous emission data indicates the absence of free-carrierinduced loss. Furthermore, our deeply-scaled down waveguide cross-section reduces the transittime of the carriers. Hence, the effective recombination lifetime has a calculated upper boundof 0.77 ns. According to Claps et al., a lifetime value below 1 ns would render the free-carrierabsorption negligible [13].
4. Conclusion
In conclusion, we have obtained significant Raman On-Off gain of 0.7 dB from 4.2 mm longsubmicron-cross-section SOI waveguides using low CW pump powers from a laser diode. TheRaman amplifier had a FOM of ∼57 dB/cm/W, approximately 10 3 greater than obtained inlarge-area Si waveguides and consistent with the low loss and small cross-section of our waveg-uide system. Further work in SOI waveguide fabrication using optimized CMOS processingtechnology can lead to even lower propagation losses, thereby allowing longer device lengthsand higher Raman gains.
Acknowledgments
This work was partially supported by DARPA/MTO University Opto Centers under ContractBROWNU-1119-24596. We thank JDS Uniphase for graciously providing optical componentsand equipment used in our testing lab. We also acknowledge helpful discussions with the JalaliGroup at UCLA and Dr. Idan Mandelbaum and Dr. Nicolae Panoiu at Columbia.
(C) 2004 OSA 9 August 2004 / Vol. 12, No. 16 / OPTICS EXPRESS 3718#4687 - $15.00 US Received 29 June 2004; Revised 20 July 2004; accepted 20 July 2004
