Newport 1621 User manual

USER’S GUIDE
Nanosecond Photodetectors
Models 1621 & 1623
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
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e-mail: [e-mail] • www.newport.com

Warranty
Newport Corporation guarantees its products to be free of defects for one
year from the date of shipment. This is in lieu of all other guarantees,
expressed or implied, and does not cover incidental or consequential loss.
Information in this document is subject to change without notice.
Copyright 2022 Newport Corporation. All rights reserved.
The New Focus logo and symbol are registered trademarks of Newport
Corporation
Document Number 90099917 Rev. A

Nanosecond Photodetectors Contents • 3
Contents
Operation 5
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Using the Photodetector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Conserving Signal Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Supplying your own load resistor. . . . . . . . . . . . . . . . . . . . . . . . 9
Checking and Replacing the Battery . . . . . . . . . . . . . . . . . . . . 10
General Features and Principles 11
Photoreceiver Circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Frequency and Time Response . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Using Filters and Optical Fiber 15
Characteristics 17
Physical Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
Photodetector Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
Customer Service 19
Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

4 • Contents
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Nanosecond Photodetectors Operation • 5
Operation
Introduction
The Model 162X is a general-purpose battery-powered
photodetector for measuring optical signals down to
nanosecond time scales.
There are two versions of the Model 162X detector,
each based on a different photodetector diode:
Model Wavelength Type Diam.
1621 350–1000 nm silicon/PIN 0.8 mm
1623 800–1700 nm InGaAs/PIN 0.1 mm
(with a 1.5-
mm ball
lens)
Complete specifications for the Model 162X
nanosecond photodetectors begin on page 17.
This product is ideal for detecting light pulses or
monitoring optical signals from DC to several hundred
MHz where an electronic amplifier is not required. The
large-diameter detectors and the DC response make
alignment and operation of the Model 162X easy.
Figure 1 shows the typical responsivity curves for the
photodetectors used in the Models 1621 and 1623.

6 • Operation
Figure 1:
Typical
responsivities
of
Model 1621
and Model
1623
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength, nm
200 400 600 800 1000 1200 1400 1600 1800 2000
Responsivity, A/W
Model 1621
Model 1623
Each Model 162X photodetector runs off a single 9-volt
alkaline battery. The photoreceiver’s slim casing,
shown below, makes it easy to position it in a set-up
between closely spaced optics. The switches and BNC
output connector are located on top of the receiver for
easy access.
Figure 2:
Model 162X
casing
Photodetector
Power switch Battery-check LED
Load-resistor switch
Output (BNC)
8-32 (M4)
Mounting
threads
A full mechanical diagram of the Model 162X casing is
available on page 17.

Nanosecond Photodetectors Operation • 7
Using the Photodetector
1. Check the battery voltage. The Model 162X is
powered by a single 9-volt alkaline battery. To
check the battery condition, push the red power
switch to the BATT CHK position. If the green LED
lights up, the battery is in good condition; if the
LED does not light, the battery needs to be
replaced (see page 10).
2. Mount the photodetector. Use the 8-32 thread (M4
for metric versions) on the bottom of the casing to
mount the photoreceiver to a post or pedestal.
The threading is seated in a non-conductive plastic
pad to reduce the electrical noise associated with
ground loops. Be careful not to over-tighten when
attaching the casing to a post or pedestal, or the
threaded insert can strip out of the plastic pad.
3. Connect the detector output. Connect your
voltmeter, oscilloscope, or other instrument to the
Output BNC connector on top of the detector.
4. Turn the power switch to “on.” The output voltage
should register on your scope or instrument.
5. Align an optical beam onto the detector. 
Be careful to keep the optical power under the
maximum optical power of 50 mW to avoid
damaging the photodetector.
6. Select the load. Use the black switch on top of the
detector to set the load to 50 , 10 k, or Open.
(Select Open if you are providing an external load
resistor. See “Supplying your own load resistor” on
page 9.)
7. Turn the detector off. When you are finished with
the detector, return the power switch to the “off”
position.

8 • Operation
Conserving Signal Strength
If you have a low-power signal, 50 of trans-
impedance gain can lead to very small output voltages.
If you need to conserve signal strength, it is important
to focus your optical beam onto the photodiode.
Adjusting the photodiode position for optimal signal is
best performed as a two-step procedure:
1. Set the load resistance to 10 kor Open.
2. Hook the detector output directly into an
oscilloscope or other voltage-measurement device
with a large input impedance.
3. Adjust the photodiode position (in both transverse
and focus directions) for maximum DC voltage.
For optical signal levels above about 1 mW (at 10
k), the voltage will saturate. Remember, the
output voltage can only rise up to the battery
voltage—9 V when the battery is fresh. If you use
the 1-Minput impedance of an oscilloscope,
saturation is even easier to reach.
4. Select the 50-resistor, or supply your own small
load resistance suitable to your gain and
frequency-response requirements. Make fine
adjustments to the photodiode position until the
maximum signal is achieved.
To compute the approximate output voltage for a given
input optical power use the relationship
Vout = Pin · R · G,
where Pin is the input optical power in watts, Ris the
photodetector’s responsivity in A/W (see page 6 for
typical responsivities), and Gis the load resistance (V/
A). For example, with 10 mW of optical power, a
responsivity of 0.5 A/W, and with the load set to 50 ,
the photodetector will produce an output voltage of
approximately (10 mW) · (0.5 A/W) · (50 V/A) = 0.25 V.

Nanosecond Photodetectors Operation • 9
Supplying your own load resistor
At high frequencies, typical resistors begin to exhibit
parasitic inductance and capacitance. For instance, if
you want to operate on the Open setting and supply
your own 100-resistor, you might be tempted to use
a standard carbon-film resistor soldered across the
terminals of a BNC tee. The problem with this
approach is that the parasitic inductance of such a
home-made resistor will cause a significant change in
the impedance at high frequencies. So, for best results
you may need to buy a precision load resistor that is
optimized for high-frequency use.
You must also watch out for impedance mismatch
problems. The internal transmission line from the
photodiode to the BNC output has a characteristic
impedance of 50 . This has been chosen for best
compatibility with 50-impedance equipment. If you
terminate this transmission line with a non-50-load,
you should expect degraded frequency response.
When you use 50-impedance equipment, you
should switch the nanosecond photodetector into the
Open position. If you leave the internal 50-resistor
switched in, the photocurrent will be divided between
the two loads, cutting your signal in half. If you leave
the internal 10-kresistor switched in, it will have little
effect, as the 50-load will draw the majority of the
photocurrent.
DC blocking devices are a potential source of
confusion. Some sensitive high frequency equipment
requires 0 V at DC input. In this case, we recommend
that you put a high-frequency DC blocking load
between the nanosecond detector and your
equipment. If so, you must supply a DC path to ground
for your photocurrent. This is easiest to do by
switching in the photodetector’s 50-internal load
resistor. If you switch in the 10-kload resistor, you

10 • Operation
must be certain that the circuit does not saturate itself
on the DC level.
Checking and Replacing the Battery
The Model 162X is powered by a single, standard 9-volt
battery. The battery lifetime depends on the load
resistor and on the optical input power to the
photodetector. Under normal operating conditions the
battery lifetime will typically be greater than 500
hours.
To check the condition of the battery, push the red
switch to the BATT CHK position. If the green LED
lights up, the battery is in good condition.
When the battery voltage falls below about 6.5 volts,
the green LED will not light up, and the battery should
be replaced.
Replacing the Battery
1. Turn the red power switch to “off” to prevent
damage to the receiver.
2. Remove the screw on the back of the
photodetector casing and remove the back cover.
3. Unplug the old battery.
4. Install a new 9-volt alkaline battery.
5. Reinstall the back cover.
6. Test the new battery’s status by pushing the power
switch to the BATT CHK position.