Newport 2007 Manual de usuario
Model 2007 & 2017 User’s Manual
Nirvana Auto-Balanced
Photoreceivers
Patent No. 5,134,276
These photodetectors are sensitive to electrostatic
discharges and could be permanently damaged if
subjected even to small discharges. Ground your-
self adequately prior to handling these detectors or
making connections. A ground strap provides the
most effective grounding and minimizes the
likelihood of electrostatic damage
phone: (877) 835-9620
e-mail: tech@newport.com • www.newport.com
2
Contents
90099912 Rev. A
I Quick Start 3
II Detector Operation 5
III Principles Of Operation 17
IV Operating With Optical Fiber 20
V Applications Examples 22
VI Troubleshooting 30
VII Warranty, Service and Support 32
VIII Specifications 33
3
This section outlines the quickest path to using the Nirvana
photoreceiver. For more detail on how to operate the Nirvana, see
Section II, Detector Operation. For more information on using the
Nirvana with fiber inputs, see Section IV, Operating With Optical Fiber.
1. Set up your optical experiment with a reference beam impinging on
the REF photodiode and your signal beam impinging on the SIGNAL
photodiode. Use an optical power between 0.1 mW and 1 mW to
facilitate oscilloscope display of your results. Make sure that your
beamsplitter produces more power in the reference beam than in
the signal beam (attenuate the signal beam if necessary).
2. Apply ±15 VDC power to Nirvana. See page 9 for more details on the
power supply connectors.
3. Switch the Linear Output knob to the SIG mode. Turn the Loop
Bandwidth knob to 100. For more detail on this function, see page 14.
4. Attach an oscilloscope to the SIGNAL MONITOR back panel BNC.
Adjust the signal beam coupling, the oscilloscope display control, and
your modulator input settings until the expected voltage waveform
display appears on the oscilloscope. For instance, if your modulator is
an optical chopper running at 1 kHz and 50% duty cycle, your output
oscilloscope trace is expected to be a square wave at 1 kHz, 50% duty
cycle, with an amplitude proportional to the received signal power.
5. Connect the oscilloscope to the back panel LINEAR OUTPUT BNC.
The displayed voltage should again be the expected replica of your
optical modulation. In the case of the chopper input, your displayed
square wave should now show some enhanced edge response.
I
Quick Start
4
6. Block the signal beam with an opaque card. Switch the Linear Output
knob to BAL. Adjust the reference beam coupling to maximize the
LINEAR OUTPUT voltage.
7. Unblock both REF and SIGNAL beams and switch the Linear Output
knob to AutoBAL.
8. If you are measuring a DC or low-frequency signal, monitor it on the
LOG OUTPUT. If your signal is at high frequencies, monitor it on the
LINEAR OUTPUT.
5
The Newport Model 2007 and 2017 Nirvana photoreceivers eliminate
many noise sources that can plague precision measurements. Nirvana’s
wide signal bandwidth and various outputs and settings give you the
flexibility to design inexpensive shot-noise limited experiments with very
little support electronics. Nirvana is designed for use in a dual-beam
setup: one invariant reference path and one signal path which contains
your experiment. Properly applied, Nirvana reduces common mode
noise by over 50 dB at frequencies from DC to 125 kHz. Thus you can
effectively eliminate laser-intensity noise and make shot-noise limited
measurements at low frequencies without using lock-in amplifiers and
optical choppers. Nirvana’s patented circuitry subtracts the reference and
signal photocurrents, canceling noise signals that are common to both
channels. Unlike conventional balanced receivers, Nirvana’s electronic
gain compensation automatically results in balanced detection, even if
the average optical intensities on the two detectors are different and
time-varying. Nirvana’s voltage outputs allow you to measure signal
power with 50 dB less noise than in a single-beam experiment. In most
cases, the noise floor will be determined by shot noise.
Nirvana is easy to use, but we recommend that you read this guide
thoroughly before you try to set up and use the detector. Before using the
photoreceiver in a fiber system, see Section IV, Operating With Optical
Fiber.
Front Panel Controls
The Model 2007 and 2017 photoreceiver inputs consist of two
photodiodes. One is designated the reference photodiode, labeled REF,
while the other is designated the signal photodiode, and labeled SIGNAL.
The outputs of the Nirvana photoreceiver are a set of user-selectable
functions of the optical power applied to these photodiodes. When
planning to use the detector in auto-balanced mode, be sure that the
reference photodiode receives more, and ideally twice as much, optical
power as the signal photodiode.
Note: Since there are no windows on the photodiode, keep the
photodiodes covered when not making measurements for extended
periods of time.
II
Detector
Operation
6
REF SIGNAL
SIGNAL
MONITOR
LINEAR
OUTPUT
LOG OUTPUT
±15 VDC
Model 2007
Nirvana Detector
125 kHz (Vis) Auto-Balanced Photorceiver
Loop
Bandwidth
Linear
Output
SIG BAL
AutoBAL
0 100
10 90
20 80
30 70
40 50 60
10X
Nirvana Back Panel
Nirvana TopPanel
Nirvana Front Panel
7
Top Panel Controls
The function of the Nirvana photoreceiver’s outputs is controlled by a
knob on the top panel. This Linear Output switch controls the operation
of the back panel LINEAR OUTPUT as summarized in the previous table.
With the Linear Output switch set to SIG, the LINEAR OUTPUT BNC
provides a voltage proportional to the signal diode’s photocurrent only.
This function is useful for monitoring the signal power when setting up
the experiment. With the Linear Output switch set to BAL the Nirvana
functions as a traditional balanced detector. The LINEAR OUTPUT BNC
provides a voltage proportional to the difference between the
photocurrents of the signal and reference diodes. Use this mode when
you want a traditional balanced receiver or to verify that the correct ratio
of signal and reference optical powers has been achieved. With the Linear
Output switch set to AutoBAL or 10X, the Nirvana automatically balances
the photocurrents from the signal and reference photodiodes at the
subtraction node. Use these modes for optimum noise cancellation. In
10X mode, the LINEAR OUTPUT BNC’s voltage is ten times greater than
in AutoBAL mode.
Knob Setting Function
SIG Signal monitor, voltage proportional to received SIGNAL optical power.
BAL Traditional balanced detection, voltage proportional to difference
between received SIGNAL and REF optical powers.
AutoBAL Autobalanced detection, zero DC voltage, noise-suppressed AC signal
proportional to received SIGNAL optical power.
10X Autobalanced detection with higher gain. Same as AutoBAL setting,
but with output voltage increased ten times.
Table 1. Back-panel LINEAR OUTPUT BNC function as determined by the top-panel Linear Output
knob position.
8
The Loop Bandwidth knob controls the cutoff frequency of the electronic
gain compensation in auto-balanced operation. In most experiments, the
optimal setting is 100. Nirvana’s auto-balancing is the result of a low-fre-
quency feedback loop that adjusts the gain of the REF photodiode to
exactly balance the SIGNAL and REF photocurrents. In some cases, it may
be advan-tageous to reduce the bandwidth of this feedback by reducing
the Loop Bandwidth knob setting. For more details on this topic see page
14 and Section III, Principles of Operation.
Back Panel Controls
The back panel of the Nirvana photoreceiver contains the power input
con-nector and three voltage output BNC’s. Power for the Nirvana is
enters through the microconnector power input, labeled ±15 VDC.
The SIGNAL MONITOR BNC directly monitors the optical beam on the
signal photodiode. This voltage output has a transimpedance gain of 10
V/mA for the signal photocurrent. This output is unaffected by the top
panel controls or the REF photodiode optical input so it can be used to
monitor the signal power. The output of the LOG OUTPUT BNC
depends on the top panel Linear Output knob setting. When it is turned
to either AutoBAL or 10X, the LOG OUTPUT voltage is:
,
otherwise the output voltage is undefined. This output voltage provides a
con-venient measurement of absorption present in the signal path. It is
bandwidth limited, with a bandwidth determined by the Top Panel Loop
Bandwidth knob, and the REF and SIGNAL incident optical powers, as
described in Section III, Principles Of Operation.
LINEAR OUTPUT is a voltage related to the REF and SIGNAL received
optical powers as determined by the top panel Linear Output knob.
LOG OUTPUT P
P
REF
SIGNAL
~ -1n −
1.
9
Making A Measurement
Mounting
Nirvana comes with a customer specified English or metric mounting
pad. Insert the mounting pad into the bottom of the detector housing
and secure with the three 4-40 screws provided. The mounting pad has a
tapped 8-32 or M4 thread for convenient mounting to a pedestal or base.
Power Requirements and Cables
Two different power cables are shipped with the photoreceiver: Model
0923 Pico (m8) double-ended, male power cable for use with the
Newport power supply, and a Model 0924 Pico (m8) male to banana plug
power cable for use with other power supplies. If you have a Newport
Model 0901 power supply, use the Model 0923 Pico (m8) double-ended,
male power cable to connect the photoreceiver to one of the power
supply’s 0.3-A microconnector outputs. Use the Model 0924 Pico (m8)
male to banana plug power cable when working with a power supply
other than the Newport Model 0901. Power supply requirements are a
minimum of 0.1 A of current on ±15V. The color convention for the
three banana plugs is:
Banana Plug Voltage Current
Red + 15 V 0.1A
Green COM/GND N/A
Black -15 V 0.1 A
Applying Optical Signals
After mounting the photoreceiver and applying power, direct the signal
and reference beams onto the respective photodetectors. You can check
the cou-pling efficiency of each signal by turning the Linear Output knob
to BAL and monitoring the LINEAR OUTPUT BNC. Successively
blocking each output
10
with a white card will allow you to optimize the coupling into each
photodetector. When you have optimized the alignment, select your
desired operating mode with the Linear Output knob.
Obtaining 50 dB Noise Suppression
To obtain the best possible noise suppression in autobalanced mode, you
must carefully design your optical experiment. This section of the
manual describes proper selection of parameters such as laser power,
power split ratio, and modulation frequency. Further, it describes several
optical component choices which may affect your perceived noise
performance.
The optical setup can greatly affect the observed noise level in your
measurement. Before beginning your experiment, measure the laser
power, the signal modulation frequency (if any) and the power split ratio
(the ratio of reference to signal power after the beamsplitter). Also
measure (or control) the optical polarization of your beams, and losses in
the signal and reference paths.
Laser Power
Figure 1 shows the common-mode rejection ratio (CMRR) vs. frequency
for different photocurrents. The split ratio was fixed at nearly 2:1.
Notice, the common-mode rejection improves with photocurrent. We
recommend received photocurrents in the mA regime, but a low
frequency CMRR of 50 dB can be obtained at microamp current levels.
Improved high-frequency performance at the lower signal levels can
be obtained through adjustment of the split ratio.
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