High-Speed Phase-Shift Modulator for GSE Lasers

The proposed innovative modulation technique that enables very high-speed modulation of SELs is possible because of the use of the grating outcoupler in GSE lasers. Instead of absorbing laser power, this technique electrically controls the efficiency at which the optical wave oscillating in the planar cavity is outcoupled normal to the surface.

The experimental (millimeter wave) far-field radiation patterns [16] from a grating in Al₂O₃ excited from both ends are shown in Fig. 15a. The millimeter (88 to 100 GHz) waveguide circuit (Fig. 15b) contained an attenuator to balance the power and a phase shifter. The radiated power is greatest if the signals are in-phase and almost zero if the signals are out-of-phase, as predicted by microwave [16] and optical theory. By adding one (or more) phase sections to a GSE laser (Fig. 16a), the outcoupled power can theoretically be nulled while maintaining a nearly constant photon density in the laser cavity. A crude experimental result of such a GSE laser is shown in Fig. 16b, which shows an L-I curve with no bias to the phase section of a GSE laser (purple curve). The blue curve corresponds to a bias of 40 mA to the gain section with bias to the phase section increasing from 0 to 30 mA (for a total input current of 70 mA).

The output power initially decreases when current is applied to the phase section and then increases as the current is further increased since the index of the phase section changes with current [17], which changes the phase of the light propagating through the phase section. In such a three terminal laser, modulation rates are not limited by the photon lifetime or by optical rise-time/fall-time constants, so low-chirp modulation in excess of 40 Gbps is predicted. This modulation approach will be developed on this program.

The theoretical radiation from a 15 µm long GSE outcoupler as a function of detuning from the Bragg condition is shown in Fig. 17 as the input phase at one end of the grating is varied, assuming that the field amplitudes are equal at both inputs to the grating. This plot, calculated using the SMU developed GRATING software, indicates that if the outcoupler is detuned ~ +/- 20 nm, the outcoupled intensity in insensitive to phase variations. However, if the outcoupler is on resonance, 100% modulation of the output beam is possible by adjusting the phase. The field distributions within the grating outcoupler corresponding to the symmetric (in-phase) and anti-symmetric (out-of-phase) condition are shown in Fig. 18. The GSE laser prototypes that Photodigm has delivered to customers have the outcoupler grating detuned ~ 17 nm so that the L-I curves remain linear even if a phase-shift should occur.

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(b)

Fig. 15. a) Experimental in-phase and out-of-phase radiation patterns for the grating-outcoupler at millimeter wave frequencies (~ 100 GHz); b) Apparatus for the millimeter wave outcoupler experiment.

(a) (b) Fig. 16. a) GSE laser with integrated phase section; b) Static L-I curve with no bias to the phase section (purple curve) and with bias applied to the phase section (blue curve).

Fig. 17. Theoretical relative intensity of light outcoupled from a 15 µm long outcoupler with a grating depth of 0.13 µm (l = 1317 µm).

            If the input amplitudes of the optical fields at each end of the grating are not equal, less power is outcoupled if both input waves are in phase, and a non-zero amount of power is outcoupled if both waves are p out of phase. Figure 19 shows in-phase and out-of-phase outcoupling for amplitude-balanced inputs and for amplitude inputs that differ by a factor of two.

        Figure 20 shows a GSE laser with an integrated phase-shift modulator with DBR sections in green, gain sections in yellow, phase-shift modulator sections in blue and the outcoupler in red. This device will be amplitude modulated, so the period of the grating outcoupler will not be detuned from the exact Bragg condition. A two wavelength, cross-grating GSE laser that incorporates such phase shifters and emits two independent wavelengths from a single aperture is shown in Fig. 21.

Fig. 18. Position of the light wave with respect to the grating teeth for a) maximum and b) zero outcoupling.

Fig. 19. Grating out-coupled light for equal incident amplitudes (red and dark blue lines) and for one wave with half the amplitude of the other (black and light blue lines).

(a) (b) Fig. 20. a) top view and b) 3D view of a GSE laser with an integrated phase-shift modulator.

Fig. 21.  Sketch of a two-wavelength cross-grating GSE laser with integrated phase-shift modulators.

      The device is symmetric to ensure that the amplitudes of the input waves to both sides of the outcoupler grating are nearly equal, for maximum outcoupling dynamic range (see Figs. 17, 18 and 19 and the accompanying discussion). For typical index changes in InP based quantum well gain sections, a phase shift of p requires a length of ~ 75 µm and a current swing from 0 to < 10 mA (consistent with the experimental data in Fig. 16b). Since the relative phase of the counter propagating waves (in the plane of the GSE laser) incident on the outcoupler will in general have a random phase, the current to one of the phase-shifters shown in Fig. 20 will be used to bias for maximum outcoupling (which will require a maximum phase shift of p/2).  High speed amplitude modulation can now be achieved by rapid variation of the current to the second phase-shifter.

        A phase shift modulated output, instead of an intensity modulated output could be obtained from such a device if the outcoupler grating was detuned ~ 17 nm.  In this case, the outcoupled intensity of the GSE laser does not change with current to the phase shift modulator (see Fig. 17), but the phase of the emitted light changes.  Such phase shift modulation can provide increased spectral efficiency, but requires sophisticated heterodyne optical receivers that have not yet been fully developed.

Present GSE lasers have demonstrated operation at 3.125 Gbps. The proposed unique phase-shift modulator can extend that rate beyond 40 Gbps. A unique advantage of the proposed modulator is that it allows low-power operation of the GSE laser at very high data rates, which is not possible with current modulated semiconductor lasers. This proposed high data rate GSE laser, especially combined with the multi-wavelength GSEs that emit from a single aperture, has the capability to greatly increase data rate, improve reliability, and reduce the cost of high performance lasers.