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MZM Alternate Laser Synthesizer (ALS)

ASIAA in collaboration with NAOJ has developed an Alternate Laser Synthesizer (ALS) based on a Mach-Zehnder Modulator (MZM) technology to generate optical Local Oscillator (LO) references that can be distributed over long distances of fiber. The standard technique for generating an optical LO is to use a master and slave laser where the difference frequency of the optical tones is used to generate the reference LO. The MZM approach employs intensity modulation on the master laser to generate optical sidebands spaced by the reference LO frequency. Both techniques have their advantages and disadvantages, however, this effort has shown that the MZM approach has inherently lower phase noise which translates to improvement in astronomical coherence. The photos in Figure 1 and Figure 2 portray the mechanical packaging of the unit.

Figure 1. MZM Laser Synthesizer engineering model. This unit was delivered
to ALMA-NA for compatibility testing with the existing Central
Local Oscillator system in August 2010.

Figure 2. Internal views of MZM Laser Synthesizer (click to enlarge). Left photo – bottom of chassis with
DC power supplies, ADAM controller (blue box), and RF power amplifier / MZM plate (lower right).
Right photo – top with optical components including EDFA (black box). Forced air cooling is
provided by 2 intake fans on the bottom and exhaust vents on the top, both via the rear panel.

A simplified block diagram of the MZM Laser Synthesizer is provided in Figure 3. The key component is the MZM device denoted by MZ1. It accepts an optical input at 1556.21 nm, +13 dBm, and provides modulation using a RF input signal over a tuning range of 13 to 31 GHz. The RF input is supplied from a low phase noise external Central Variable Reference (CVR), which is then amplified by a Centellax power amplifier, AR1, followed by Bias-T for DC injection, then to port-C of MZ1.

Figure 3. Simple block diagram of the MZM Laser Synthesizer. A master laser at a wavelength of
1556.21 nm is modulated by MZ1 using a RF reference tone generated by the Central
Variable Reference. The desired optical output tones are depicted in the lower right.

Figure 4. Internal configuration of Mach-Zehnder Modulator device, MZ1. This device
consists of 3 separate sub-MZMs on a single substrate. Sub-MZMs A and B are used for fine
phase control of the optical signals to sub-MZM C. Sub-MZM C receives both DC bias for mode
control and RF for phase modulation. The combination of the upper and lower waveguide paths
produces intensity modulation on the output port

An illustration of the MZM device is provided in Figure 4. The optical input is split into two optical paths and recombined after applying the phase modulation. The interferometer is composed of a Lithium Niobate substrate with a refractive index that changes with the applied electrical field and is the basis of the phase modulation. At the output, complete destructive interference will result with an applied phase of 180 degrees across one arm with respect to the other. MZ1 is composed of 3 such interferometric arms, each one being controlled by separate DC biases, supplied at ports A, B and C. The RF input to the MZM is supplied at port C at a level of +28 dBm along with the DC bias via Bias-T, BT1. The MZM Laser Synthesizer operates in one of two modes:

    Full Bias mode: In this mode the modulated light goes through two Fiber Bragg Gratings (FBG), FB1, and FB2, which notch out the central laser tone. The sidebands used in this mode are the second harmonics, and is achieved by applying 0 Volts to MZ1-C to retain power in the zeroth and the even harmonics. As a result, the LO produced is 4 x RF, which is the difference between the upper and lower sidebands of the second harmonic. This mode provides LOs for all bands except Band 1 and Band 5.
    Null Bias mode: In this mode Vπ is applied to port C to suppress the central laser tone, where Vπ is defined as the half wave voltage. Vπ for MZ1 is 5V, and in general depends on the material, size of the device as well as other parameters. Only the odd harmonics survive in this mode, and the difference between the upper and lower sidebands of the first harmonic generates the LO which is 2 x RF. This mode supplies the LO for Band 1 and Band 5. The selection between Full bias and Null bias mode is performed by optical switch SW1.

The modulation efficiency of MZ1 reduces with increasing RF frequency and results in a lower modulation index. This variation in modulation index with RF frequency results in an optical output power disparity of ~14 dB across a CVR frequency range of 13 to 31 GHz. In order to meet the output power requirement of +3 dBm +/- 1 dB per tone, gain leveling was employed into the Erbium Doped Fiber Amplifier (EDFA), AR2. The EDFA introduces a large noise pedestal that degrades the phase noise performance of the MZM Laser Synthesizer. A band pass filter, FL3, removes a significant portion of the optical noise floor and results in a worst case optical output SNR of 20 dB at the highest operating frequency. All the components and optical fibers are Polarization Maintaining (PM) in order to meet the Polarization Extinction Ratio (PER) requirement.

A typical output performance of the MZM Laser Synthesizer at 100 GHz is shown in Figure 5. The unit is operated in Full Bias mode with an input RF reference of 25 GHz. The resulting optical output tones are spaced apart by exactly 100 GHz. A spectral density plot of the 100 GHz RF tone, L(f), is shown in the right panel with an integrated phase noise value of 1.1 degrees RMS (integrated from 3 kHz to 3 MHz). This number is inclusive of phase noise contributions from the CVR and the MZM Laser Synthesizer. The residual phase noise for the MZM Laser Synthesizer alone (after removing the CVR contribution) is 0.4 degrees RMS.

Figure 5. Performance at 100 GHz. Left – optical output with tones spaced apart by 100 GHz;
Center – 100 GHz tone at photomixer output; Right – phase noise plot showing total integrated phase noise of
1.1 degrees RMS and a residual phase noise of 0.4 degree RMS (spec is 1.0 degrees RMS).

The MZM Laser Synthesizer is controlled via a simple web interface shown in the Figure 6. The first three links in the figure control the basic operations of the unit. The “Set LO” link allows the user to set the LO frequency (27 – 122 GHz) and automatically calculates and sets the mode of operation (Null or Full Bias), MZ1 DC biases, and EDFA gain. The optical tones are generated within 50 milliseconds of the command initiation. The “Unit Health” link leads to the page shown in Figure 7 that provides continuous monitoring of the RF power from AR1, regulated voltages, and 7 internal temperature sensors located within the chassis.

Figure 6. Main Web interface allows simple GUI commands to control and monitor the MZM Laser Synthesizer unit.

Figure 7. Unit Health page monitors various parameters within the unit, including
RF power to MZ1 port-C, regulated voltages, and various component temperatures located
within the unit.

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