Setting the LD Current

Safety Recommendations

Overview

The laser diode current is controlled by two potentiometers:

BIAS - Sets the lower limit (threshold current)
GAIN - Sets the upper limit (maximum working current)

These work together with the modulation voltage (0.3V-5V) to provide linear current control.

Current Range by Model

Model	Current Range
iLD500	10 - 500 mA
iLD2500	70 - 2500 mA
iLD4500	140 - 4500 mA

Setup Procedure

Step-by-Step Instructions

1. Initialize Potentiometers

Turn both BIAS and GAIN screws counterclockwise (25 turns) to zero.

2. Connect Laser Diode

Connect the laser diode to the Output terminal on the board.

3. Set Supply Voltage

Choose appropriate supply voltage: 5-24 VDC

Formula:

V_SUPPLY = V_LD + V_DROP + margin

Where:

V_LD = Laser diode forward voltage
V_DROP = ~2V (driver voltage drop)
margin = Additional headroom

Example:

Laser forward voltage: 5V
Supply voltage: 5V + 2V = 7-9V recommended

4. Connect Power and Modulation

Connect the main power supply to the Input terminal
Connect the modulation source to the Input terminal

5. Enable the Driver

Make sure the Interlock loop is closed
Turn On the main supply voltage
Reset the start signal (if not already reset)
Set the start signal to enable the laser output
Wait for the 5-second power-up delay to complete

6. Set BIAS Current (Threshold)

Set modulation voltage to 0.2-0.3 VDC (minimum)

Adjust BIAS current:

Turn the screw of the BIAS potentiometer clockwise
Increase slowly until the threshold current of the laser diode is reached
Laser should just start to lase

7. Set GAIN Current (Maximum)

Set modulation voltage to 5 VDC (maximum)

Adjust GAIN current:

Turn the screw of the GAIN potentiometer clockwise
Increase until the maximum working current is reached
Monitor the current using measurement points (see Measuring the LD Current)

8. Verify BIAS Current

Set modulation voltage back to 0.2-0.3 VDC

Check BIAS current:

Measure current (see measurement procedure)
If it has changed from step 6, adjust counterclockwise to restore original value

This is necessary because adjusting GAIN can affect BIAS.

9. Verify GAIN Current

Set modulation voltage back to 5 VDC

Check GAIN current:

Measure current
If it has changed from step 7, adjust clockwise to restore desired value

10. Final Verification

Repeat steps 8 and 9 if necessary until both BIAS and GAIN are stable.

The laser diode driver is now set.

Current vs. Modulation Voltage

Linear Relationship

I_LD = I_BIAS + (V_MOD - 0.3V) × (I_GAIN - I_BIAS) / 4.7V

Where:

I_LD = Laser diode current
I_BIAS = Current at V_MOD = 0.3V
I_GAIN = Current at V_MOD = 5V
V_MOD = Modulation voltage (0.3V - 5V)

Example Calculation

Settings:

BIAS = 50 mA (@ 0.3V)
GAIN = 500 mA (@ 5V)

At V_MOD = 2.65V:

I_LD = 50 + (2.65 - 0.3) × (500 - 50) / 4.7
I_LD = 50 + 2.35 × 450 / 4.7
I_LD = 50 + 225
I_LD = 275 mA

Current Control Characteristics

Modulation Transfer Function

V_MOD (V)   I_LD (mA)
0.0-0.2  →  0 (laser off)
0.3      →  BIAS
1.0      →  BIAS + 15% of range
2.0      →  BIAS + 36% of range
3.0      →  BIAS + 57% of range
4.0      →  BIAS + 79% of range
5.0      →  GAIN (maximum)

Setup Flow Diagram

Initialize
├─ Set BIAS = 0
└─ Set GAIN = 0
        ↓
Connect LD
        ↓
Set Supply Voltage
(V_LD + 2V to V_LD + 7V)
        ↓
Connect Power & Modulation
        ↓
Enable Driver
(Close interlock, set start signal)
        ↓
Set BIAS
├─ V_MOD = 0.3V
└─ Adjust BIAS to threshold
        ↓
Set GAIN
├─ V_MOD = 5V
└─ Adjust GAIN to max current
        ↓
Verify BIAS
├─ V_MOD = 0.3V
└─ Re-adjust if changed
        ↓
Verify GAIN
├─ V_MOD = 5V
└─ Re-adjust if changed
        ↓
Done ✓

Important Safety Notes

Heat Dissipation

Heat sink requirements:

Adequate thermal mass
Good thermal contact with driver board
Sufficient airflow (if needed)
Monitor driver temperature

Protection During Setup

✅ Start with both potentiometers at zero
✅ Connect laser before applying power
✅ Use Standard Mode for manual control
✅ Increase current slowly and carefully
✅ Monitor laser optical output
✅ Stay within laser specifications
✅ Use laser safety goggles

Common Mistakes to Avoid

Mistake	Consequence	Prevention
Applying power before connecting LD	Laser damage	Always connect LD first
Excessive supply voltage	Driver/LD damage	Keep V_SUPPLY - V_LD < 7V
No heat sink at high current	Overheating	Install heat sink for I > 300mA
Not re-checking BIAS after GAIN	Incorrect threshold	Always verify BIAS after GAIN adjustment

Recommended Current Settings

Conservative Settings (Long Life)

Set maximum current to 80-90% of laser rated current:

Extends laser lifetime
Reduces thermal stress
Improves reliability

Nominal Settings

Set maximum current to 100% of laser rated current:

Normal operating point
Balanced performance and lifetime

Modulation Input Configuration

Voltage Source Requirements

Parameter	Specification
Voltage Range	0.3V - 5V
Input Impedance	~20 kΩ
Threshold	0.2-0.3V
Bandwidth	DC - 500 kHz (sine)
	DC - 100 kHz (TTL)

Signal Sources

Compatible modulation sources:

Function generator (analog)
DAC output
PWM (with filtering)
Analog control voltage
Microcontroller DAC

Troubleshooting Setup Issues

Laser Won’t Lase

Possible Causes:

BIAS set too low
Modulation voltage below threshold
Laser diode damaged
Wrong polarity

Solutions:

Increase BIAS gradually
Verify modulation voltage >0.3V
Check laser diode
Verify connections

Unstable Current

Possible Causes:

Poor power supply regulation
Noise on modulation input
Thermal issues

Solutions:

Use regulated, filtered supply
Shield modulation input
Improve cooling

Current Won’t Reach Target

Possible Causes:

Insufficient supply voltage
GAIN potentiometer at limit
Driver limitation

Solutions:

Increase supply voltage (within limits)
Verify driver model matches current requirement
Check for faults