All-silicon raman amplifiers and lasers based on micro ring resonators

Devices for generating a laser beam are disclosed. The devices include a silicon micro ring having at least one silicon optical waveguide disposed at a distance from the micro ring. The radius and the cross-sectional dimension of the microring, the cross-sectional dimension of the waveguide, and the distance between the micro ring and the waveguide are determined such that one or more pairs of whispering gallery mode resonant frequencies of the micro ring are separated by an optical phonon frequency of silicon. Methods of manufacturing a lasing device including a silicon micro ring coupled with a silicon waveguide are also disclosed.

FIELD OF THE INVENTION

The present invention relates to optical amplification and lasing devices, and methods for manufacturing the devices. More particularly, the present invention relates to low-threshold microcavity Raman lasers, and methods for manufacturing the same.

BACKGROUND OF THE INVENTION

Stimulated Raman scattering (“SRS”) has a rich and evolving history since the development of the laser. In 1962, SRS effect at infrared frequencies was discovered. This discovery was soon described as a two-photon process with a full quantum mechanical calculation. To account for anti-Stokes generation and higher-order Raman effects, however, coupled-wave formalism was adopted to describe the stimulated Raman effect. Self-focusing was later included to account for the much larger gain observed in SRS. These understandings facilitated the study and design of Raman amplifiers and lasers. For example, low-threshold microcavity Raman lasers have been demonstrated in silica micro spheres and micro disks using excited whispering gallery modes (“WGMs”). Such devices can play an important role in the developing technology of photonic integrated circuits.

Because silicon is being considered as a promising platform for photonic integrated circuits, silicon based photonic devices have been increasingly researched. Microscopic passive silicon photonic devices such as bends, splitters, and filters have been developed. Active functionalities in highly integrated silicon devices have been studied, such as optical bistability due to the nonlinear thermal-optical effect and fast all-optical switching with two-photon absorption.

Silicon based Raman amplifiers and lasers also have been studied. The bulk Raman gain coefficient gRin silicon is 104times higher than in silica. Light generation and amplification in planar silicon waveguides with Raman effects have been studied recently. Raman lasing using a silicon waveguide as the gain medium has been demonstrated, where the ring laser cavity is formed by an 8-m-long optical fiber. A Raman laser using an S-shaped 4.8-cm-long silicon waveguide cavity with multi-layer coatings has also been reported, which could be integrated onto CMOS-compatible silicon chips.

Despite these advances, microscopic low-threshold Raman amplification and lasing devices on a monolithic silicon chip has yet to be developed. Such devices would support the development towards efficient, all-optical photonic integrated circuits.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide all-optical on-chip signal amplification and lasing. In particular, embodiments of the present invention include Raman amplification and lasing devices using on-chip micro ring resonators coupled with waveguides in monolithic silicon. Embodiments of the present invention also provide methods for manufacturing such devices. According to embodiments of the present invention, lasers are designed with geometries so that WGM resonant frequencies of the micro ring resonator match the pump-Stokes frequency spacing of SRS in monolithic silicon. Therefore, one or more pairs of pump and Stokes light can form WGMs in the micro ring resonator.

Devices for generating a laser beam are disclosed. In some embodiments, the devices include a silicon micro ring having a radius and a cross-sectional dimension, and at least one silicon optical waveguide having a cross-sectional dimension and disposed at a distance from the micro ring. The distance, the radius, and the cross-sectional dimensions are determined so that at least one pair of whispering gallery mode resonant frequencies of the micro ring are separated by an optical phonon frequency of silicon.

Methods of manufacturing a lasing device including a silicon micro ring coupled with a silicon waveguide are disclosed. In some embodiments, the methods include determining a radius and a cross-sectional dimension of the micro ring, a cross-sectional dimension of the waveguide, and a distance between the micro ring and the waveguide, so that at least one pair of whispering gallery mode resonant frequencies of the micro ring are separated by an optical phonon frequency of silicon. The methods also include manufacturing the lasing device by creating the micro ring with the determined radius and cross-sectional dimension, creating the waveguide with the determined cross-sectional dimension, and disposing the micro ring from the waveguide at the determined distance.

DETAILED DESCRIPTION OF THE INVENTION

Devices of various embodiments of the present invention use micro ring resonators as a cavity for producing Raman laser. The strong light confinement of a micro ring resonator enhances the stimulated Raman scattering with low-threshold pump power.

FIG. 1ais a top view andFIG. 1bis a cross-sectional view of a Raman amplification and lasing device of various embodiments of the present invention, generally at100. Device100includes an optical micro ring resonator102and an optical waveguide104, both made of silicon. The cross-sectional view illustrates a cross section of device100taken by a plane perpendicular to the sheet and passing through the center of micro ring resonator102.

In some embodiments, waveguide104can be a quasi-transverse electric (“quasi-TE”) single-mode waveguide. As shown, micro ring resonator102has radius R. Micro ring resonator102and waveguide104can have the same width w and height tsi. There is a gap s between resonator102and waveguide104. Resonator102and waveguide104can be formed on top of a layer of silicon oxide (SiO2)106. In some embodiments, device100can include more than one waveguide disposed at a close distance to resonator102. Although waveguide104is shown to be straight inFIG. 1, it can assume other shapes as known in the art.

In operation, pump light can enter the lower end109of waveguide104in the direction of arrow108. This pump light can induce pump light in resonator102in the direction of arrow110by a coupling effect. When resonator102is stimulated, Stokes light in resonator102is generated by Raman scattering, which causes Stokes light leaving the upper end of waveguide104by the coupling effect.

Stimulated Raman scattering in micro ring resonator102is a two-photon process related to the optical phonons. The strongest Stokes peak arises from single first-order  Raman-phonon (three-fold degenerate) at the Brillouin zone center. The coupling between the pump and Stokes lightwaves in SRS can be described by Maxwell's equations using nonlinear polarizations p(3):

The nonlinear polarization Ps(3)is cast as χjkmn(3)EpE*pEs, where χjkmn(3)is the third-order fourth-rank Raman susceptibility, and can be calculated in terms of the Raman tensorRi. The Epand the Esare electric fields of the pump and Stokes waves respectively.

In micro ring resonator102, it can be shown that the cavity SRS enhancement results from the intensity build up in the cavity, so that the threshold pump power depends on the quality factor Q and also the coupling efficiencies. The intensity build up factor for the cavity mode is:
Ic/I0=(Qλ)/(π2nR)  (3)
where I0is the intensity of the input light, Icis the effective intensity of the light in the cavity, λ is the light wavelength, n is the refraction index of the micro ring resonator102host material, and R is the radius of the micro ring resonator102. The effective interaction length is:
Lc=(Qλ)/(2πn)  (4)

Both pump mode and Stokes mode can be WGMs with quality factors Qpand Qs, respectively. The condition for Raman lasing is that the gain exceeds the losses:
gRξIc,pump>Lc,Stokes−1(5)

Assume that the modal volume is Vm≈2πRA, the threshold pump power pth=I0A is:

Pth=π2⁢n2ξ⁢⁢gR⁢QS⁢Qp⁢Vmλp⁢λS(6)
where the parameter ξ<1 describes the coupling to the pump mode and the overlap between the pump and Stokes modes, A is the effective cross-sectional area of the cavity mode, and gRis the bulk Raman gain coefficient of silicon, which is about 70 cm/GW for Stokes radiation in the 1550-nm range.

Because the quality factors QSand Qpof pump mode and Stokes mode are relatively high, threshold power Pthcan be made very low. Therefore, by designing a highly confined micro ring resonator102that supports pump and Stokes modes, a microscopic low-threshold on-chip amplification and lasing device100can be fabricated.

The following describes the design of device100that supports one or more pump and Stokes modes. Device100can be designed by numerically solving Maxwell's equations (1) and (2) with a boundary condition corresponding to the geometry of device100, using a three-dimensional finite-difference time-domain (3D FDTD) method. With a 3D FDTD method, a transmission spectrum of device100, resonant wavelengths, WGM field profiles, and quality factor Q of the resonant wavelengths can all be calculated. This can be performed with any software that numerically solves the Maxwell's equations (1) and (2), such as the FullWAVE™ software provided by RSoft Design Group, Inc. (Ossining, N.Y.).

An important goal of the design is to determine iteratively (i.e., fine-tune) the geometry of device100so that WGM resonant frequencies of micro ring resonator102corresponds to one or more pairs of pump and Stokes frequencies. A pump frequency and a corresponding Stokes frequency are spaced apart byΔv=15.6 THz, which is the optical phonon frequency in monolithic silicon. If a pair of WGM resonant frequencies are separated by 15.6 THz, a pump light having one of the pair of frequencies can be used to generate a Stokes light having the other frequency, and both the pump and the Stokes light can form WGMs in resonator102.

If it is desirable that device100supports pump and Stokes lights with wavelengths close to a predetermined wavelength (e.g., 1550 nm), the geometry of device100can be determined iteratively so that wavelengths corresponding to the WGM resonant frequencies of resonator102are close to the predetermined wavelength (e.g., within the range of about 1400 nm to about 1600 nm). However, device100is not limited by the example provided; device100can also be designed to support pump and Stokes lights with wavelengths within other suitable ranges.

According to various embodiments of the present invention, a numerical design process can include determine iteratively the geometry of device100and calculating the corresponding transmission spectrum of waveguide104with, for example, a 3D FDTD method. The drops in the transmission spectrum correspond to WGM resonant frequencies of resonator102. From the transmission spectrum, a pump wavelength λpcan be chosen, such that λpcorresponds to a drop in the transmission spectrum. Then, the Stokes wavelength can be calculated with λS=λp+λp2/(c/Δv−λp). Stokes wavelength λSshould also correspond to a drop in the transmission spectrum. Quality factors Qpand QScan then be calculated with Q=λ/ΔλFWHMfrom the transmission spectrum.

Determining iteratively the geometry of device100can include determining iteratively the radius R of resonator102, the width w and the height tSiof waveguide104and micro ring resonator102, and the gap s between waveguide104and resonator102, so that the transmission spectrum of waveguide104have certain desired properties. For example, width w and height tSican be changed it to shift the high Q resonant spectrum of device100to a range close to 1550 nm. A starting point for the iterative determination of width w and height tSican be values that support a quasi-TE single-mode waveguide104. Radius R can be determined iteratively so that optical phonon frequency (15.6 THz) is an integer multiple of the free spectral range, which is the spacing between the neighboring WGM resonant frequencies of resonator102(the WGM resonant frequencies corresponds to drops in the transmission spectrum of the waveguide). Gap s can be determined iteratively to achieve a good electromagnetic coupling efficiency into and out of resonator102for different wavelength ranges.

As an example, device100can be designed with w equals to 350 nm, tSiequals to 200 nm, s equals to 150 nm, and R equals to 4.9 μm. In this example, the cross-sectional dimension of waveguide104as represented by w and tSisupports a quasi-TE single-mode. Height toxideof SiO2layer106can be 400 nm. The refraction index of silicon and SiO2can be nSi=3.48 and noxide=1.46 respectively.

FIG. 2illustrates the quasi-TE transmission spectrum near 1550 nm of waveguide104coupled with resonator102according to this example design. As shown, two pump lights (Pump1and Pump2) can be selected from the transmission spectrum, with wavelengths λp1=1431.8 nm and λS1=1546.5 nm. Two corresponding Stokes lights (Stokes1and Stokes2) have wavelengths λp2=1444.8 nm and λS2=1562.5 nm, which also correspond to drops of the transmission spectrum.

FIG. 3illustrates the WGMs of Pump1inside micro ring resonator102of this example with continuous wave excitation. The white dots302in resonator102are the locations having stronger Hy field (magnetic field in the Y direction). It can be seen that Pump1forms WGMs in resonator102. The WGMs of Pump1in resonator102are caused by Pump1in waveguide104traveling in the direction of arrow108(Z direction).

FIG. 4illustrates the WGMs of Stokes1inside micro ring resonator102of this example with continuous wave excitation. The white dots402in resonator102are the locations having stronger Hy field (magnetic field in the Y direction). It can be seen that Stokes1forms WGMs in resonator102as well. The WGMs of Stokes1in resonator102are caused by Stokes1in waveguide104traveling in the direction of arrow108(Z direction).

Therefore, device100, according to the example design, supports the WGMs of both Pump1and Stokes1. By SRS and coupling, Pump1in waveguide104can induce WGMs of both Pump1and Stokes1in micro ring resonator102, and hence Stokes1in waveguide104. Similarly, device100, according to the example design, supports the WGMs of both Pump2and Stokes2. It should be noted that the example geometry of device100is not the only geometry that can support WGMs of the required pump and Stokes frequencies.

FIG. 5is a flow chart illustrating various processes for manufacturing Raman amplification and lasing devices of various embodiments of the present invention. At500, a suitable geometry of device100is determined. At502, a layer of polymethylmethacrylate (“PMMA”) can be coated on top of a silicon-on-insulator (“SOI”) wafer. For example, a 200 nm thick 495 495K A6 PMMA can be spin-coated on top of a SOI wafer. At504, a design pattern according to the determined geometry can be written on the PMMA layer by electron-beam lithography. At506, the exposed PMMA layer can be developed in a solution. For example, a solution of methylbutylisoketone (“MIBK”) and isopropyl alcohol (“IPA”) with MIBK:IPA=1:3 can be used to develop the PMMA layer for about 55 seconds. At508, a chrome mask can be transferred on top of the SOI wafer by thermal evaporation. At510, the SOI wafer can be etched to form the designs in the wafer, using, for example, inductively coupled plasma (“ICP”) etching. At512, the chrome mask can be removed. The wafer can be further packaged to seal the optical devices fabricated on the wafer.FIG. 6is a top view of a fabricated Raman amplification and lasing device of various embodiments of the present invention captured by scanning electron microscopy (“SEM”).

Other embodiments, extensions, and modifications of the ideas presented above are comprehended and within the reach of one skilled in the art upon reviewing the present disclosure. Accordingly, the scope of the present invention in its various aspects should not be limited by the examples and embodiments presented above. The individual aspects of the present invention, and the entirety of the invention should be regarded so as to allow for modifications and future developments within the scope of the present disclosure. The present invention is limited only by the claims that follow.