New Focus 1621 User manual

USER’S GUIDE
Nanosecond
Photodetectors
Models 1621 & 1623
5215 Hellyer Ave. • San Jose, CA 95138-1001 • USA
phone: [phone] • fax: [phone]
e-mail: [e-mail] • www.newfocus.com
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Warranty
New Focus, Inc. 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 2001, 1999, New Focus, Inc. All rights reserved.
The symbol and NEW FOCUS, Inc. are registered trademarks of
New Focus, Inc.
Document Number 162107 Rev. B
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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
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4 • Contents NEW FOCUS, Inc.
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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:
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.
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)
Note:
Note:
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6 • Operation NEW FOCUS, Inc.
Figure 1:
Typical
responsivities
of
Model 1621
and 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
A full mechanical diagram of the Model 162X casing is
available on page 17.
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
Photodetector
Power switch Battery-check LED
Load-resistor switch
Output (BNC)
8-32 (M4)
Mounting
threads
Note:
Note:
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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 dam-
aging the photodetector.
6. Select the load. Use the black switch on top of the
detector to set the load to 50 Ω , 10 k Ω , 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.
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8 • Operation NEW FOCUS, Inc.
Conserving Signal Strength
If you have a low-power signal, 50 Ω of transimped-
ance 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. Adjust-
ing the photodiode position for optimal signal is best
performed as a two-step procedure:
1.
Set the load resistance to 10 k Ω or Open .
2.
Hook the detector output directly into an oscil-
loscope or other voltage-measurement device
with a large input impedance.
3. Adjust the photodiode position (in both trans-
verse and focus directions) for maximum DC
voltage.
For optical signal levels above about 1 mW (at 10
k Ω ), the voltage will saturate. Remember, the out-
put voltage can only rise up to the battery volt-
age—9 V when the battery is fresh. If you use the
1-M Ω input impedance of an oscilloscope, satura-
tion is even easier to reach.
4.
Select the 50- Ω 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
V
out
= P
in
· R · G ,
where P
in
is the input optical power in watts, R is the
photodetector’s responsivity in A/W (see page 6 for
typical responsivities), and G is 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 Ω ,
the photodetector will produce an output voltage of
approximately (10 mW) · (0.5 A/W) · (50 V/A) = 0.25 V.
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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- Ω resistor, you might be tempted to use
a standard carbon-film resistor soldered across the ter-
minals 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 imped-
ance 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 Ω . This has been chosen for best
compatibility with 50- Ω impedance equipment. If you
terminate this transmission line with a non-50- Ω load,
you should expect degraded frequency response.
When you use 50- Ω impedance equipment, you
should switch the nanosecond photodetector into the
Open position. If you leave the internal 50- Ω resistor
switched in, the photocurrent will be divided between
the two loads, cutting your signal in half. If you leave
the internal 10-k Ω resistor switched in, it will have lit-
tle effect, as the 50- Ω load will draw the majority of the
photocurrent.
DC blocking devices are a potential source of confu-
sion. 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 equip-
ment. 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- Ω internal load resistor. If you
switch in the 10-k Ω load resistor, you must be certain
that the circuit does not saturate itself on the DC level.
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10 • Operation NEW FOCUS, Inc.
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 photode-
tector. 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 photode-
tector 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.
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Nanosecond Photodetectors General Features and Principles • 11
General Features and Principles
Photoreceiver Circuitry
The circuitry inside the Model 162X consists of a
reverse-biased photodetector and a load resistor with
three settings: 50 Ω , 10 k Ω , and Open . The 50- Ω set-
ting has 50-V/A sensitivity and can be used to achieve
the ≤ 1-nanosecond response time. The 10-k Ω setting
has a reduced time response but gives a higher sensi-
tivity (10,000 V/A) and can be used for alignment pur-
poses and for detecting low-power pulses. The Open
setting allows you to hook-up your own external load
resistor and choose the best gain-bandwidth combina-
tion for your application.
Figure 3:
Functional
schematic of
the Model 162X
circuitry
Frequency and Time Response
The nanosecond photodetector circuitry is quite
straightforward, consisting only of the photodiode,
battery bias and a selectable load resistor. The speed
limitation of the circuit is established by the junction
Output
Open
10 kΩ50 Ω
ON
+V
LED
Battery Check
Circuit
OFF
Batt
Chk
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12 • General Features and Principles NEW FOCUS, Inc.
capacitance of the photodiode and the load resistance
value. Figures 4 and 5 give typical frequency responses
and pulse responses for the Model 162X
photodetector.
Figure 4:
Typical
frequency
response for
the Model 1623
on the 50- Ω
setting
When performing high-speed measurements, imped-
ance-matching effects between your measurement
apparatus and the photo-detector must be taken into
consideration in order to achieve optimal high-speed
response.
Typical laboratory measurement equipment may pre-
vent you from achieving nanosecond response times.
BNC connectors do not match 50- Ω transmission lines
well for frequencies above 100 MHz. The frequency
response is therefore not flat between 100 MHz and
500 MHz.
Above 500 MHz, the situation can be worse, with typi-
cal BNC connectors and cables proving to be quite
lossy. Therefore, while the time response of the nano-
second detector is quite fast, it is not expected to pro-
vide flat frequency response in the 100-MHz to 500-
MHz regime.
Selecting the internal 50- Ω resistor will usually allow
you to achieve a response speed which is limited by
your detection equipment. Typical laboratory
0 100 200 300 400 500 600
Frequency, MHz
Gain (10 dB/div)
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Nanosecond Photodetectors General Features and Principles • 13
oscilloscopes, for instance, have only 10-MHz to 100-
MHz bandwidth. When you select the 10-k Ω resistor,
the speed is RC limited, and the response speed will
typically be ≥ 50 ns.
Figure 5:
Typical pulse
response for
the (a) Model
1621 and (b)
Model 1623
photodetectors
0246810
Time (ns)
(a) Model 1621
0246810
Time (ns)
(b) Model 1623
A 0.15-ps pulse at 1.06 µm was used to test the
photodetectors, and the output was measured
using a 12-GHz oscilloscope.
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14 • General Features and Principles NEW FOCUS, Inc.
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Nanosecond Photodetectors Using Filters and Optical Fiber • 15
Using Filters and Optical Fiber
New Focus offers accessories, sold separately, to attach
a 1"- diameter filter or an optical fiber to the detector.
Both accessories attach to the casing using the 1.04-32
threads located in the casing around the detector. Note
that the accessories are also compatible with two other
New Focus products, the Model 203X large-area pho-
toreceiver and the Model 215X femtowatt photore-
ceiver.
The Model 1280 1" filter holder allows you to mount a
1"- diameter optic in front of the photodetector. For
instance, you can mount a colored glass filter to
remove unwanted wavelengths or a neutral-density
filter to attenuate the optical power incident on the
photodetector. The Model 1280 has a plastic ring for
mounting a filter that is up to about 0.25" (6.4-mm)
thick. A thicker optic can be held in place using the
6-32 nylon-tipped set screw. Use a 1/16" or 1.5-mm
Allen key or ball-driver to adjust the set screw.
The Model 1281 FC fiber adapter allows you to connect
an FC connectorized fiber to the front of the photode-
tector. The coupling efficiency of light from an optical
fiber to the Model 1623 photodetector is typically 70%
or greater. The coupling efficiency is less than unity
because of the small detector size and small variations
in the mechanical position of the detector and the
fiber.
See Figure 6 for drawings of these two accessories. A
variety of fiber couplers, fiber collimators and pigtails
are also available from New Focus.
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16 • Using Filters and Optical Fiber NEW FOCUS, Inc.
Figure 6:
Model 1280
1" filter holder
and the
Model 1281 FC
fiber adapter
0.63"
(15.9)
Model 1280
patent pending
dia. 1.30"
(33.0)
holes for
tightening
1.04-32
thread
Retaining ring
for holding 1"
or 25-mm optics.
6-32 nylon-tipped
setscrew for
holding 1" or
25-mm optics.
0.13"
(3.2)
FC connector
holes for
tightening
1.04-32
thread
0.35"
(8.9)
0.19"
(4.8)
Model 1281
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Nanosecond Photodetectors Characteristics • 17
Characteristics
Physical Specifications
Figure 7:
Mechanical
drawing of the
Model 162X
casing
4.03
(102.3)
1.00
(25.4)
1.25 (31.8)
1.040-32
Threaded hole
1.16
(29.5)
Battery check LED
2.50 (63.5)
Output (BNC)
Load resistor switch
Power switch
8-32 (M4)
Threaded insert
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18 • Characteristics NEW FOCUS, Inc.
Photodetector Specifications
Model # 1621 1623
Wavelength Range 350-1000 nm 800-1700 nm
Detector Material/Type Silicon/PIN InGaAs/PIN
Detector Diameter 0.8 mm 0.1 mm (with 1.5-
mm-diameter ball
lens)
Typical Max. Responsivity 0.55 A/W (at 800 nm) 1.0 A/W (at 1600 nm)
Maximum Optical Power 50 mW 50 mW
Load Resistors 50 Ω, 10 kΩ, and
Open (user provided)
50 Ω, 10 kΩ, and
Open (user provided)
Typical Rise Time ≤1 nanosecond (50-
Ωsetting)
≤1 nanosecond (50-Ω
setting)
Typical detector capacitance 3 pF 0.8 pF
Electrical Output Connector BNC BNC
Power Requirements One 9-volt alkaline
battery
One 9-volt alkaline
battery
Typical Battery Lifetime >500 hours >500 hours
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Nanosecond Photodetectors Customer Service • 19
Customer Service
Technical Support
Information and advice about the operation of any
New Focus product is available from our applications
engineers. For quickest response, ask for “Technical
Support” and know the model and serial numbers for
your product.
Hours: 8:00–5:00 PST, Monday through Friday
(excluding holidays).
Toll Free: 1-866-NUFOCUS (1-866-683-6287)
(from the USA & Canada only)
Phone: [phone]
Support is also available by fax and email:
Fax: [phone]
Email: [e-mail]
We typically respond to faxes and email within one
business day.
Service
In the event that your photodetector malfunctions or
becomes damaged, please contact New Focus for a
return authorization number and instructions on
shipping the unit back for evaluation and repair.
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20 • Customer Service NEW FOCUS, Inc.
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