Measuring the LD Current

Overview

The measurement of the laser diode current can be done by measuring the reference voltage at the points above the shunt resistor while the laser diode is connected.

This provides a non-invasive method to monitor the actual laser current during operation.

Measurement Points Location

The measurement points are located above the shunt resistor on the driver board.

    ┌─────────────┐
    │   Measure   │
    │   Points    │
    └──────┬──────┘
           │
    ┌──────┴──────┐
    │    Shunt    │
    │  Resistor   │
    └─────────────┘
           │
        To Laser

Measurement Procedure

Required Equipment

Digital multimeter (DMM)
- Voltage measurement capability
- Minimum 3½ digit resolution
- DC voltage range selection

Step-by-Step Measurement

Ensure laser diode is connected to the driver
Power up the driver and enable laser output
- Close interlock
- Set start signal (if required)
- Apply modulation voltage
Set multimeter to DC voltage mode
- Select appropriate range (typically 200mV or 2V)
- Ensure DC coupling
Connect multimeter probes to measurement points
- Red probe: Upper measurement point (positive)
- Black probe: Lower measurement point (negative/ground)
Read voltage displayed on multimeter
Calculate current using the conversion ratio for your model

Voltage-to-Current Conversion

The conversion ratio is model-dependent based on the shunt resistor value.

Conversion Ratios

Model	Conversion Ratio	Calculation
iLD500	1.0 V/A (1:1)	I(A) = V(V) × 1
iLD2500	0.2 V/A (1:5)	I(A) = V(V) × 5
iLD4500	0.1 V/A (1:10)	I(A) = V(V) × 10

Formula by Model

iLD500 (10-500mA)

I_LD (A) = V_measured (V) × 1
I_LD (mA) = V_measured (V) × 1000

iLD2500 (70-2500mA)

I_LD (A) = V_measured (V) × 5
I_LD (mA) = V_measured (V) × 5000

iLD4500 (140-4500mA)

I_LD (A) = V_measured (V) × 10
I_LD (mA) = V_measured (V) × 10000

Worked Examples

Example 1: iLD500

Measured voltage: 0.04 V

Calculation:

I_LD = 0.04 V × 1 = 0.04 A = 40 mA

Result: Laser current is 40 mA

Example 2: iLD2500

Measured voltage: 0.04 V

Calculation:

I_LD = 0.04 V × 5 = 0.20 A = 200 mA

Result: Laser current is 200 mA

Example 3: iLD4500

Measured voltage: 0.04 V

Calculation:

I_LD = 0.04 V × 10 = 0.40 A = 400 mA

Result: Laser current is 400 mA

Example 4: High Current (iLD4500)

Measured voltage: 0.35 V

Calculation:

I_LD = 0.35 V × 10 = 3.50 A = 3500 mA

Result: Laser current is 3500 mA (3.5 A)

Measurement Range by Model

Expected Voltage Ranges

Model	Current Range	Voltage Range
iLD500	10 - 500 mA	0.01 - 0.5 V
iLD2500	70 - 2500 mA	0.014 - 0.5 V
iLD4500	140 - 4500 mA	0.014 - 0.45 V

Quick Reference Table

iLD500 (1:1 ratio)

Voltage (V)	Current (mA)
0.010	10
0.050	50
0.100	100
0.200	200
0.300	300
0.400	400
0.500	500

iLD2500 (1:5 ratio)

Voltage (V)	Current (mA)
0.014	70
0.100	500
0.200	1000
0.300	1500
0.400	2000
0.500	2500

iLD4500 (1:10 ratio)

Voltage (V)	Current (mA)
0.014	140
0.100	1000
0.200	2000
0.300	3000
0.400	4000
0.450	4500

Measurement Best Practices

Accuracy Considerations

Factors affecting measurement accuracy:

Multimeter accuracy (typically ±0.5% to ±2%)
Contact resistance at measurement points
Shunt resistor tolerance
Temperature effects on shunt resistance

Expected accuracy: ±2-5% typical

Improving Accuracy

✅ Use quality digital multimeter
✅ Ensure good probe contact
✅ Allow system to stabilize thermally
✅ Take multiple readings and average
✅ Calibrate against known current source (if available)

Measurement Tips

Alternative Measurement Methods

External Current Meter

You can also measure current by inserting a current meter in series with the laser diode:

Oscilloscope Measurement

For dynamic current measurements:

Connect oscilloscope probe to measurement points
Set to DC coupling
Use appropriate voltage range
Apply conversion ratio to waveform

Advantages:

View current waveform in real-time
Measure modulation depth
Analyze rise/fall times
Detect current ripple

Troubleshooting Measurements

Zero or Very Low Voltage

Possible Causes:

Laser not enabled
No modulation voltage
Poor probe contact
Measurement points not connected

Solutions:

Verify laser is enabled (Laser LED ON)
Check modulation voltage >0.3V
Clean measurement points
Ensure good probe contact

Voltage Higher Than Expected

Possible Causes:

Current set too high
Wrong conversion ratio used
Meter on wrong range

Solutions:

Verify GAIN setting
Check driver model and use correct ratio
Select appropriate meter range

Unstable Reading

Possible Causes:

Modulation signal noise
Poor probe contact
Thermal effects
Power supply ripple

Solutions:

Stabilize modulation source
Improve probe contact
Allow thermal stabilization
Check power supply quality

Reading Doesn’t Match Expected

Possible Causes:

Wrong model assumed
BIAS/GAIN not set correctly
Calculation error

Solutions:

Verify driver model number
Re-set BIAS and GAIN
Double-check conversion ratio

Safety Precautions

During Measurement

✅ Ensure laser safety precautions (goggles, enclosure)
✅ Avoid touching high-voltage points
✅ Keep hands clear of laser beam path
✅ Monitor laser temperature
✅ Watch for overheating indicators

Continuous Monitoring

For applications requiring continuous current monitoring:

Data Logger Connection

Connect a data acquisition system to measurement points:

Use high-impedance input (>1MΩ)
Sample at appropriate rate
Apply conversion ratio in software
Log to file for analysis

Panel Meter Installation

Install analog or digital panel meter:

Connect across measurement points
Calibrate using conversion ratio
Provides visual indication
Can add alarm outputs

Documentation

Recording Measurements

Document current measurements including:

Date and time
Modulation voltage
Measured voltage
Calculated current
Operating conditions (temperature, etc.)
Any observations

This creates a valuable operating history for:

Troubleshooting
Trend analysis
Laser aging assessment
Performance verification