Dietmar Wahl

Polarization-stable single-mode VCSELs for Cs-based MEMS atomic clock applications

Vertical-cavity surface-emitting lasers (VCSELs) emitting at 894.6 nm wavelength have been fabricated for Cs-based atomic clock applications. For polarization control, a previously developed technique relying on the integration of a semiconducting surface grating in the top Bragg mirror of the VCSEL structure is employed. More specifically, we use a so-called inverted grating. The VCSELs are polarized orthogonal to the grating lines with no far-field diffraction side-lobes for sub-wavelength grating periods. Orthogonal polarization suppression ratios exceed 20 dB. The polarization stability has been investigated at different elevated substrate temperatures up to 80 °C, where the VCSEL remains polarization-stable even well above thermal roll-over. For the purpose of integration with the atomic clock microsystem, flip-chip-bondable VCSEL chips have been realized. Sub-mA threshold currents and sufficient output powers in the milliwatt range are achieved. The required modulation bandwidth of more than 5 GHz is reached at only 0.5mA bias. Maximum bandwidths above 10 GHz have been measured even at elevated temperatures up to 80 °C. Modulation current efficiency factors larger than 12 GHz/√mA are achieved at room temperature. Moreover, the intrinsic modulation characteristics of the VCSELs are investigated by precise curve fitting of measured small-signal modulation response curves and relative intensity noise spectra. A K-factor of less than 0.4 ns and a maximum 3 dB bandwidth exceeding 22 GHz are obtained.

Polarization-stable vertical-cavity surface-emitting lasers with inverted grating relief for use in microscale atomic clocks

Vertical-cavity surface-emitting lasers (VCSELs) with single-mode, single-polarization emission at a wavelength of 894.6 nm have become attractive light sources for miniaturized Cs-based atomic clocks. So far, VCSELs used for these applications are single-mode because of small active diameters which has the drawbacks of increased ohmic resistance and reduced lifetime. By employing surface grating reliefs, enhanced fundamental-mode emission as well as polarization-stable laser oscillation are achieved. VCSELs with 5 μm active diameter show side-mode suppression ratios of 20 dB even at currents close to thermal roll-over with orthogonal polarization suppression ratios better than 20 dB at elevated ambient temperatures up to 100 °C.

Polarization Control and Dynamic Properties of VCSELs for MEMS Atomic Clock Applications

We report the polarization stability and small-signal characteristics of single-polarization single-mode vertical-cavity surface-emitting lasers (VCSELs) emitting at 894.6-nm wavelength for Cs-based miniature atomic clock applications. Polarization control is achieved by integration of a semiconducting surface grating in the top Bragg mirror. The VCSELs are polarized orthogonal to the grating lines with a peak-to-peak difference between the dominant and the suppressed polarization modes reaching 29 dB even at substrate temperatures up to 65°C . A modulation bandwidth of more than 5 GHz is reached at only 0.5-mA bias. Modulation current efficiency factors larger than 12 GHz/√(mA) are achieved. Moreover, the intrinsic modulation characteristics of the VCSELs are investigated. A K-factor of less than 0.30 ns and a maximum 3-dB bandwidth exceeding 30 GHz are obtained.

True bidirectional optical interconnects over multimode fiber

We report the fabrication and properties of 850nm wavelength AlGaAs/GaAs-based transceiver chips, in which vertical-cavity surface-emitting lasers (VCSELs) and photodiodes are monolithically integrated. Various types of devices allow half- and full-duplex bidirectional optical interconnection at multiple Gbit/s data rates over a single butt-coupled glass or polymer-clad silica optical fiber with core diameters of 100 or 200 μm. Whereas metal-semiconductor-metal (MSM) photodiodes are employed for these large-area fibers, we also investigate the integration of PIN-type photodiodes which appear more promising in combination with standard 62.5 or 50 μm core diameter graded-index multimode fibers. This interconnect solution based on two identical chips is attractive owing to lower volume, weight, and cost. Applications will be found in home, in-building, industrial, or automotive networks and potentially within computer clusters or central offices.

Fabrication and Characterization of Low-Threshold Polarization-Stable VCSELs for Cs-Based Miniaturized Atomic Clocks

Vertical-cavity surface-emitting lasers (VCSELs) with single-mode, single-polarization emission at a wavelength of 894.6 nm have been fabricated for Cs-based atomic clock applications. For polarization control, monolithically integrated surface gratings are employed. Simulated and experimental results show that the longitudinal position of the surface grating has a significant influence on the threshold current. With a grating in the topmost in-phase layer, the threshold currents are reduced to 40% compared to earlier atomic clock grating VCSELs with inverted structure. The output polarization is parallel to the grating lines with a peak-to-peak difference between the dominant and the suppressed polarization modes of 25 dB even at substrate temperatures up to 80 °C. Small-signal modulation characteristics of grating VCSELs are presented. The modulation bandwidth exceeds the required 5 GHz at a bias current of only 0.9 mA above threshold at room temperature. A revised chip design with smaller mesa size and thinner passivation layer has been implemented. The VCSELs show similar electrical parasitic bandwidths but require fewer fabrication steps and hence have reduced processing complexity. K-factors of less than 0.4 ns corresponding to a maximum 3-dB bandwidth exceeding 25 GHz are obtained at 80 °C ambient temperature.

Metrological characterization of custom-designed 894.6 nm VCSELs for miniature atomic clocks

We report on the characterization and validation of custom-designed 894.6 nm vertical-cavity surface-emitting lasers (VCSELs), for use in miniature Cs atomic clocks based on coherent population trapping (CPT). The laser relative intensity noise (RIN) is measured to be 1 × 10(-11) Hz(-1) at 10 Hz Fourier frequency, for a laser power of 700 μW. The VCSEL frequency noise is 10(13) · f(-1) Hz(2)/Hz in the 10 Hz < f < 10(5) Hz range, which is in good agreement with the VCSEL’s measured fractional frequency instability (Allan deviation) of ≈ 1 × 10(-8) at 1 s, and also is consistent with the VCSEL’s typical optical linewidth of 20-25 MHz. The VCSEL bias current can be directly modulated at 4.596 GHz with a microwave power of -6 to +6 dBm to generate optical sidebands for CPT excitation. With such a VCSEL, a 1.04 kHz linewidth CPT clock resonance signal is detected in a microfabricated Cs cell filled with Ne buffer gas. These results are compatible with state-of-the-art CPT-based miniature atomic clocks exhibiting a short-term frequency instability of 2-3 × 10(-11) at τ = 1 s and few 10(-12) at τ = 10(4) s integration time..

Monolithic VCSEL–PIN Photodiode Integration for Bidirectional Optical Data Transmission

We present the monolithic integration, properties, and operation of 850-nm wavelength AlGaAs/GaAs-based vertical-cavity surface-emitting lasers (VCSELs) and PIN (p-doped-intrinsic-n-doped) photodetectors. The stacked layer structure requires sophisticated fabrication methods, but enables the use as, e.g., low-cost transceiver (TRx) devices. The TRx chips reported here are especially designed for bidirectional short-reach optical data links using a single butt-coupled standard multimode fiber (MMF). Photodiodes (PDs) with three different epitaxial layer structures are investigated. Devices with a 3-μm-thick intrinsic region show a responsivity of >0.6 A/W and have the lowest dark currents and highest 3-dB bandwidths of around 8 GHz. The maximum small-signal bandwidth of the VCSEL is 12.5 GHz. The parasitics of both devices are extracted by modeling the reflection spectra from S-parameter measurements. Investigations regarding the mutual influence between the closely integrated devices in full-duplex operation are carried out. The optical crosstalk is below -11 dB and the maximum electrical crosstalk between VCSEL and PIN PD of around -50 dB is nearly negligible. The butt-coupled MMF with a core diameter of 50 μm allows maximum fiber alignment tolerances in the range of 14-26 μm. Data transmission in the 10-Gb/s range is demonstrated in half-duplex and full-duplex mode.

Bidirectional Multimode Fiber Interconnection at Gb/s Data Rates With Monolithically Integrated VCSEL–PIN Transceiver Chips

We present the monolithic design, fabrication, and properties of 850-nm wavelength AlGaAs-GaAs-based transceiver chips for low-cost bidirectional optical data transmission over a butt-coupled standard multimode fiber of a few hundred meter length. The chips with a stacked layer structure of a vertical-cavity surface-emitting laser (VCSEL) and a PIN (p-doped-intrinsic-n-doped) photodetector can well handle data rates of 9 Gb/s in back-to-back mode and 7 Gb/s over a 500-m-long 50-μm core diameter fiber.

Single-Fiber Bidirectional Optical Data Links with Monolithic Transceiver Chips

We report the monolithic integration, fabrication, and electrooptical properties of AlGaAs-GaAs-based transceiver (TRx) chips for 850 nm wavelength optical links with data rates of multiple Gbit/s. Using a single butt-coupled multimode fiber (MMF), low-cost bidirectional communication in half- and even full-duplex mode is demonstrated. Two design concepts are presented, based on a vertical-cavity surface-emitting laser (VCSEL) and a monolithically integrated p-doped-intrinsic-n-doped (PIN) or metal-semiconductor-metal (MSM) photodetector. Whereas the VCSEL-PIN photodiode (PD) chips are used for high-speed bidirectional data transmission over 62.5 and 50 μm core diameter MMFs, MSM TRx chips are employed for 100 or 200 μm large-area fibers. Such a monolithic transceiver design based on a well-established material system and avoiding the use of external fiber coupling optics is well suited for inexpensive and compact optical interconnects over distances of a few hundred meters. Standard MMF networks can thus be upgraded using high-speed VCSEL-PIN transceiver chips which are capable to handle data rates of up to 10 Gbit/s.

Monolithic integration of VCSELs and PIN photodiodes for bidirectional data communication over standard multimode fibers

We present the monolithic design, fabrication and properties of 850nm wavelength AlGaAs-GaAs-based transceiver chips with a stacked layer structure of a VCSEL and a PIN photodetector. Bidirectional data transmission via a single, two-side butt-coupled multimode fiber (MMF) is thus enabled. The approach aims at a miniaturization of transceiver chips in order to ensure compatibility with standard MMFs with core diameters of 50 and 62.5μm used predominantly in premises networks. These chips are supposed to be well suited for low-cost and compact half- and full-duplex interconnection at Gbit/s data rates over distances of a few hundred meters.