Vescent Photonics D2 Series User manual

Offset Phase
Lock Servo
Vescent Photonics, Inc.
www.vescentphotonics.com
4865 E. 41st Ave
Denver, CO 80216
Phone [phone]
Fax [phone]

General Warnings and Cautions
The following general warnings and cautions are applicable to this instrument.
WARNING
This instrument is intended for use by qualified personnel who
recognize shock hazards or laser hazards and are familiar with
safety precautions required to avoid possible injury. Read the
instruction manual thoroughly before using to become familiar
with the instrument’s operations and capabilities.
CAUTION
There are no serviceable parts inside the instrument. Work
performed by persons not authorized by escent Photonics may
void the warranty.
CAUTION
Although ESD protection is designed into the instrument,
operation in a static-fee work area is recommended.
WARNING
To avoid electrical shock hazard, connect the instrument to
properly earth-grounded, 3-prong receptacles only. Failure to
observe this precaution can result in severe injury or death.
WARNING
Do not clean outside surfaces of any escent Photonics products
with solvents such as acetone. Front panels on electronics
modules may be cleaned with a mild soap and water solution.
Do not clean optics modules.

Limited Warranty
escent Photonics warrants this product to be free from defects in materials and workmanship for a period of one year
from the date of shipment. If this product proves defective during the applicable warranty period, escent Photonics, at
its option, either will repair the defective product without charge or will provide a replacement in exchange for the
defective product. The customer must notify escent of the defective product within the warranty period and prior to
product return. The customer will be responsible for packaging and shipping the defective product back to escent
Photonics, with shipping charges prepaid.
escent Photonics shall not be obligated to furnish service under this warranty from damage caused by service or repair
attempts made without authorization by escent Photonics; from damage caused by operation of equipment outside of
its specified range as stated in either the product specification or operators manual; from damage due to improper
connection to other equipment or power supplies.
This warranty is in lieu of all other warranties including any implied warranty concerning the suitability or fitness of
the product for a particular use. escent Photonics shall only be liable for cost of repairs or replacement of the
defective product within the warranty period. escent Photonics shall not be liable for any damages to persons or
property resulting from the use of the product or caused by the defect or failure of this product. escent Photonics'
liability is expressly limited to the warranty set out above. By accepting delivery of this product, the purchaser
expressly agrees to the terms of this limited warranty.
escent Photonics
Printed Jul 11, 2012

Absolute Maximum Ratings
Note: All modules designed to be operated in laboratory environment
Parameter Rating
Environmental Temperature >15°C and <30°C
Environmental Humidity <60%
Environmental Dew Point <15°C

Vescent Photonics D2 Series Product Manual
1. Offset Phase Lock Servo
Model No. D2-135
Document Revision: 2
1.1. Description
The D2-135 Offset Phase Lock Servo (OPLS) locks the frequency and phase of the input beat-note to a
(multiple of the) reference frequency. The beat-note is typically generated by overlapping two lasers (a
master laser and a slave laser), whose interference forms a beat-note, which is a measurement of the
frequency difference (or offset) between the two lasers. The beat-note is either input as an electrical signal
via an SMA electrical connector (D2-135-SMA) or as an optical signal via a fiber (D2-135-FC). The
reference frequency can be sourced internally from voltage controlled oscillator ( CO) or an externally
from a user-input frequency reference. The output of the OPLS adjusts the frequency of the slave laser to
lock the frequency offset between the two lasers.
The beat-note input to the OPLS can be divided by N = 8, 16, 32, or 64 -- as set by the user – before its
frequency and phase is compared to the reference frequency. The locked frequency offset is given by the
formula:
Offset = N x Reference Frequency.
By adjusting either the reference frequency or N, the frequency difference between the lasers can be
precisely adjusted. The offset frequency is adjustable from 250 MHz to up to 10 GHz.
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Vescent Photonics D2 Series Product Manual
A schematic of the OPLS, along with the D2-150 Heterodyne module is shown in Figure 1.The key
component in the OPLS is a phase-frequency detector (PFD). The PFD compares the phase and frequency
of the divided-by-N beat-note to the reference frequency. The PFD outputs a signal proportional to the
phase difference between the two input frequencies when there are no phase-slips between the two
signals. This output provides a true phase-lock error signal. When there are phase slips, the PFD acts as a
frequency comparator, aiding initial lock-up and enabling the OPLS to function as a frequency offset lock
for laser sources with significant phase noise such as DFB and DBR laser diodes. The output of the PFD
is fed to a charge pump and finally to the loop filter, where it is then fed back to the slave laser to control
the frequency of the beat-note.
The loop filter has user-adjustable proportional-integral-differential (PID) feedback and an additional
high-frequency roll-off frequency. Tuning the values of the PID loop filter allow the user to optimize the
feedback to the laser for best offset locks. This is further discussed in Section 1.7.
The OPLS can be controlled via the front-panel, or with a computer via a 9 pin D-sub connector and BNC
connections on the back of the OPLS. In this way, the frequency offset can be jumped and tuned via an
external computer.
The D2-150 Heterodyne Module is designed for use the with D2-135-FC.
1.2. Options
The D2-135 comes in two different versions:
•D2-135-SMA – SMA input of the electrical beat-note
•D2-135-FC-800 – Fiber-input of optical beat-note for wavelengths 750nm – 870nm
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Figure 1: Schematic of the D2-135 Offset Phase Lock Servo and
D2-150 Heterdyne Module

Vescent Photonics D2 Series Product Manual
The D2-135-SMA requires an electrical signal input between 10 and -10 dBm of power.
The D2-135-FC-800 includes a 10 GHz GaAs high-speed photo-detector for converting the input optical
beat-note into a electrical beat-note. The maximum optical input power is 1 mW. Warning input powers
over 1mW of optical power can damage the OPLS. A minimum of 50 μW of power in the optical beat-
note is required1.
1 Note the power in the beat-note is not the same as total power. Because the optical input uses multi-mode fiber, the
spatial mode overlap between the two lasers is not guaranteed. If the overlap between the two lasers is poor, there
can be very little power in the beat-note despite large optical power.
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Vescent Photonics D2 Series Product Manual
1.3. Specifications
Value Units
Min Offset Fre uency2<250 MHz
Max Offset Fre uency Min: 9, Typical: 103GHz
VCO drift 500 ppm/°C
PFD Noise4-213 dBc/Hz
ωI (Integral/Prop. zero) 4,8,16,32,64,125,250,500 kHz
ωD (Proportional/diff. zero) 16,32,64,125,250,500,1000,off kHz
ωHF (high freq. cutoff pole) 0.25, 0.5, 1,2,4,8,16,off MHz
Electronics Input / Output Impedance 50 Ω
User Adjustable Gain50 -50 dB
Max Electronic Beat-Note Input (D2-135-SMA) 10 dBm
Min Electronic Beat-Note Input (D2-135-SMA) -10 dBm
Max Optical Beat-Note Input (D2-135-FC) 1 mW
Min Optical Beat-Note Input (D2-135-FC)650 μW
2 Low frequency beat-notes are possible with the D2-135-SMA provided the low-frequency beat-note is a square-
wave and not sine-wave input and that the loop bandwidth ~10 times lower than your reference frequency.
3 Maximum Offset Frequency depends on power of input beat-note signal.
4 See Section 1.8. for a full description
5 Not including internal gain compensation for N divider. See Section 1.6. for more details.
6Approximate value as exact value depends on wavelength of the light and spatial overlap between the lasers.
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Vescent Photonics D2 Series Product Manual
1.4. Modes
The OPLS operates in 12 different modes, controlled by the front-panel knob or with external inputs to
the back-panel. These 12 modes select the value of the divide-by-N (N=8,16,32 or 64) and select between
three states for the reference frequency (external input, internal CO High, internal CO Low). The
table below shows the offset frequency ranges for these 12 different modes.
Divide-by-N settings
N=8 N=16 N=32 N=64
Reference
Fre uency
Setting
External 250 – 1,920 480 – 3,840 960 – 7,680 1,920 – 10,000
VCO Low 385 - 850 770 – 1,700 1,540 – 3,400 3,080 – 6,800
VCO High 770 – 1,700 1,540 – 3,400 3,080 – 6,800 6,160 – 10,000
Table 1: Offset frequency ranges for the 12 modes. All frequencies are in MHz.
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Vescent Photonics D2 Series Product Manual
1.5. Inputs, Outputs, and Controls
1.5.1. Monitor Section
Located at the top of the front panel, the monitor section contains 6 BNC outputs for monitoring various
signals used by the OPLS.
Error Signal (DC)
This is a heavily filtered monitor of the charge pump (CP) output. When the laser is phase-
locked, the CP output is proportional to phase error between the divided beat-note and the
reference signal given by
N∗11 mV
deg ∗θ
, where θ is the phase-error in degrees. . When the laser
is not phase-locked, the output is
±4 ∗(1−fsmall
2fbig )
where fsmall is the desired lock frequency or
the actual beat-note frequency, whichever is smaller and fbig is whichever is bigger. The sign of
the output depends on whether the beat-note is larger or smaller than the desired lock frequency
and the gain sign. Note that depending on the noise on the beat-note, the output can very quickly
jump back and forth between these two modes making Error Signal (DC) difficult to interpret.
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Figure 2: Schematic drawing of the front and back panels.

Vescent Photonics D2 Series Product Manual
Error Signal (HF)
This is an unfiltered version of Error Signal (DC) that is four times smaller, and has a DC offset.
When phase locked, the output is
1.6 V+2.8 mV
deg ∗θ
, where θ is the phase-error in degrees.
When in frequency mode, the output is
1.6 V±1V∗(1−fsmall
2fbig )
Ref Fre
REF FREQ provides an AC coupled, buffered monitor of the reference frequency going into the
PFD. Signal should be terminated with 50Ω.
Servo Out
The SER O OUT monitors the output signal that is fed back to the laser.
Ramp
RAMP is a monitor for the ramp signal sent to the SER O OUTPUT when the laser is in RAMP
mode.
Ramp TTL
The RAMP TTL is a trigger synchronous with the ramp. Typically it is used to trigger an
oscilloscope while sweeping the SER O OUTPUT. The RAMP TTL signal is also available on
the back panel as a dedicated trigger output.
1.5.2. Front Panel
Power (LED indicator)
All electronic modules have a blue LED power indicator on the top right side of the front panel
control section. The LED requires +15 and -15 in order to light.
Offset Mode Selector
This 12 position rotary switch selects the value of N (N=8,16,32 or 64) and the reference oscilator
mode (External, CO Low, CO High). See section 1.4 for more details. The mode selected can
be overridden with the computer control.
VCO Tune (10-turn potentiometer)
CO TUNE adjusts the control voltage of the internal CO, thereby adjusting the frequency of
the CO. The frequency of the CO can be monitored with the REF FREQ Monitor when in a
mode that uses this internal CO.
Ramp Amp (1-turn knob)
The RAMP AMP sets the amplitude of the internal ramp generator.
Optical Input (SC Fiber Input) – D2-135-FC only
Optical beat-note input for the D2-135-FC.
Beat-note Input (SMA Electrical Input) – D2-135-SMA only
Electronic beat-note input for the D2-135-SMA.
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Vescent Photonics D2 Series Product Manual
Gain Sign (two-position switch)
The GAIN SIGN reverses the sign of the charge pump (CP) output. Effectively this selects
whether the OPLS locks the slave laser to a frequency above or below the master laser frequency.
Laser State (three-position switch)
The lock switch has three positions. The lowest is the RAMP, which connects a triangle-wave to
the SER O OUTPUT, causing the laser to sweep. The amplitude of the sweep is controlled with
RAMP AMP knob. In the center position (UNLOCK) zero volts is output to SER O OUTPUT.
In the top position (LOCK) the loop filter is engaged.
Fine Gain (1-turn knob)
The FINE GAIN control adjusts the loop filter's proportional gain from 0 - 10 dB.
Coarse Gain (eight-position switch)
The COARSE GAIN sets the overall proportional gain of the circuit without changing the
location of any zeros or poles in the loop filter transfer function. The coarse gain adjusts in steps
of 6 dB from 0 dB to -42 dB.
Servo Output
The SER O OUTPUT is a voltage output to control the frequency of (typically) the slave laser.
When the OPLS is used with the D2-105 Laser Controller, the SER O OUTPUT is connected to
the CURRENT SER O INPUT on the Laser Controller. The SER O OUTPUT is the output
from the loop filter when in LOCK mode, zero volts when in UNLOCK mode, and a DC
balanced triangle wave when in RAMP mode.
Feed Forward Input
FEED FORWARD INPUT is a high impedance input that gets summed in with the SER O
OUTPUT. It is designed to provide feed forward when jumping the offset frequency of the laser.
For example, if the laser is jumped from an offset of 3 GHz to 6 GHz, the servo output needs to
jump by a consistent voltage (e.g. 2 ). This 2 jump can be applied to the FEED FORWARD
INPUT when the offset frequency is jumped. Since the loop filter does not need to integrate the
full 2 change in output, the loop obtains a phase-lock much faster after jumping the offset
frequency.
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Vescent Photonics D2 Series Product Manual
1.5.3. Right Side Panel
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Vescent Photonics D2 Series Product Manual
The feedback loop is defined by the Gain vs Frequency plot shown above. ωI, ωD and ωHF define three
zeros in the transfer function. ωI is the frequency where the gain switches from having integral gain to
having proportional gain. ωD is the frequency where the gain switches from proportional to differential.
ωHF is the frequency where the gain begins to fall off at high frequency. ωI, ωD and ωHF are each controlled
by a rotary switch.
Integrator (ωI)
Sets the frequency of the first integrator (ωI). This knob selects between values between 4 kHz
and 500 kHz.
Differential (ωD)
Sets the frequency of the differentiator (ωD). This knob selects between values between 16 kHz
and 1 MHz and off (no differential gain).
Differential Gain (25-turn trimpot)
Sets the maximum differential gain. 25-turn trimpot adjusts the gain from 5 dB to 15 dB.
Integrator (ωHF)
Sets the frequency of the high-frequency roll-off (ωHF). This knob selects between values between
250 kHz and 16 MHz and off.
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Figure 3: Schematic of the OPLS right-side panel, showing the
configurable transfer function and its user-controls.

Vescent Photonics D2 Series Product Manual
Ramp Master / Slave (Not Shown)
This jumper is only accessible by removing the right side panel and sets whether the ramp input is
in master or slave mode. It is factory set to be in MASTER MODE. In SLA E MODE (jumper
off) the RAMP signal is generated externally and input through the back panel RAMP I/O port. In
MASTER MODE (jumper on) the ramp is generated internally and is sent out to the RAMP I/O
port for driving other D2-135 OPLS or D2-125 Laser Servos configured in SLA E MODE.
1.5.4. Back-panel Section
Power I/O (9-pin D-sub)
Each electronics module is powered through a 9-pin D-sub connector through either a power
bridge unit or a serial cable with 9-pin D-sub connectors, which is convenient when the unit must
be moved for access to the side panels. The pin outs are shown in the following figure:
Computer Control (9-pin D-sub)
The 9 pin COMPUTER CONTROL signals allows an external computer to override the front-
panel and set the mode of the OPLS. All signal pins float high and can be driven by a open drain
or open collector. The pin definitions are shown below:
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Vescent Photonics D2 Series Product Manual
Pin 1 must be pulled to ground to enable computer control. Otherwise, all other pins on this cable
are ignored and the OPLS state is set by the front-panel rotary switch. Pin 2 controls whether the
gain sign is positive (pin 2 high) or negative (pin 2 low). For pins 3 through 6, refer to the tables
below:
Pins 7 and 8 are not used.
Ref. Fre . In
When the OPLS uses an external frequency reference, this input is used to supply the frequency
reference. The input is AC coupled and 50 Ω terminated. Max power is 10 dBm.
Beat-note Monitor
(÷2)
This is a digitized (i.e. square-wave) version of the input beat-note after a divide-by-2. For
example, if the input beat note is 6 GHz, the monitor will have a 3 GHz output. The signal is ~0
dBm in power regardless of the strength of the input beat-note signal.
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N2
Low High
N1 Low N=8 N=32
High N=16 N=64
Table 2: Computer Control response to
changing N1 (pin 5) and N2 (pin 6)
when Front Panel Active (pin 1) is low.
VCO External
Low High
VCO High
Range
Low CO Low External
High CO High External
Table 3: Computer control response to changing
CO External (pin 3) and CO High Range (pin
4) when Front Panel Active (pin 1) is low.

Vescent Photonics D2 Series Product Manual
Ramp TTL
Same as the front panel signal. The RAMP TTL is a trigger synchronous with the ramp. It is
used to trigger an oscilloscope while sweeping the SER O OUTPUT.
Ramp IN / OUT
The OPLS is shipped with the RAMP in MASTER MODE. In this configuration, the RAMP IN /
OUT is an output of the maximum ramp signal, generated internally. By removing the side panel
the RAMP MASTER/SLA E jumper can be accessed. When this jumper is removed, the OPLS
is in slave mode. In this configuration, RAMP IN / OUT is an input of an external ramp signal.
When the LASER STATE is in RAMP MODE, the SER O OUTPUT is an attenuated version of
the input to RAMP IN / OUT. The attenuation is controlled by the RAMP AMP knob. If
controlling multiple lasers with multiple D2-125's or D2-135 to sync the ramps, one of the servos
must be in master mode and the rest in slave mode with all the RAMP IN / OUT signals
connected. In this way each laser will sweep off a synced signal and only one oscilloscope trigger
is needed for all the lasers. Furthermore, a function generator can be used to input a ramp at a
different frequency than the 500 Hz ramp used in the OPLS.
VCO Fre . Adjust
This input is summed in with the CO TUNE potentiometer to set the voltage to the CO, and
thus the reference frequency when the OPLS is using the internal CO. The impedance to this
input in 1 kΩ and can accept voltages from -10 to +10 and should tune over entire CO
range, provided that the CO TUNE potentiometer is set in the middle of the CO range.
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Vescent Photonics D2 Series Product Manual
1.6. Understanding Gain in the OPLS
As described in Error Signal (DC) in 1.5.1 of page 12, the charge pump (CP) output is proportional to
phase-error when phase-locked, but the slope is proportional to the value of N. This means that when N is
increased, the same voltage on the CP monitor reflects twice as much phase-error. The result is that the
input to the loop filter has half as much gain with N=16 than with N=8. To compensate, the OPLS
internally increases the gain of the Loop Filter by a factor of N so the user goes not see a change in the
closed-loop gain when changing the value of N. Note that when the laser is not phase-locked, the gain
does change as the value of N changes.
1.7. Understanding the Transfer Function
The charge pump in the OPLS outputs a signal proportional to the phase-error and the transfer function is
as described in section 1.5.3. However, the OPLS will typically be used to control a frequency-tunable
device (such as a laser). In this configuration, the effective loop filter is not the one shown in Figure 3, but
includes a extra integration corresponding to converting the phase-error input to a frequency error. Thus,
ωI sets the frequency transition from single-integration to double-integration and ωI from single-
integration to proportional feedback. It is important to understand this 'hidden' integrator when
configuring the loop filter parameters.
1.8. Calculating Phase Noise
The phase-noise specified in Section 1.3 is referenced to the phase frequency detector (PFD) at 1 Hz. To
convert that to the noise measured on the actual beat-note, it must be rescaled with the following formula:
where N is the value of the divider and FREF is the reference frequency as measured in Hz. For more
details, please see http://www.vescent.com/2012/calculating-phase-noise-from-the-d2-135/ .
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D2−135 Phase−NoiseFloor=−213+20Log(N)+10 log (FREF)