How much light can a VCSEL produce at a given drive current? And at what point does it actually begin lasing?
These questions are easier to answer when you have light-current-voltage (L-I-V) readings. Instead of relying on a single optical power value, engineers can see how a laser responds as the current increases, where lasing begins, and how much electrical input is needed to reach a particular output level.
A VCSEL L-I-V curve connects three basic measurements: optical power, drive current, and forward voltage.
The supplied graph illustrates this relationship. Its optical output begins rising noticeably near 0.8 mA and reaches approximately 2.1 mW at 3 mA. The accompanying voltage trace rises more gradually, approaching 2.03 V at the highest plotted current.
These readings provide a useful starting point for understanding threshold current, slope efficiency, and electrical-to-optical efficiency.
What Is a VCSEL L-I-V Curve?
L-I-V stands for Light-Current-Voltage. It describes how a vertical-cavity surface-emitting laser behaves as electrical current passes through its semiconductor structure.
A VCSEL emits light perpendicular to the semiconductor wafer surface. Its compact structure and surface emission make it suitable for optical sensing, precision photonics, and integrated laser assemblies.
The L-I-V characteristics include two relationships.
The first is the light-current relationship, which shows how optical output changes with drive current.
The second is the voltage-current relationship, which shows the forward voltage required at different current levels.
Reading these together helps engineers understand the electrical and optical behavior of the device.
The Three Parameters Behind the Curve
The relationship between these parameters is not perfectly linear across the entire current range.
Below threshold, optical output is relatively weak. Once lasing begins, optical power rises more rapidly.
At sufficiently high current, self-heating may eventually reduce the rate of optical power growth.
Reading the VCSEL L-I-V Curve
The supplied graph shows optical power and forward voltage as the drive current increases from 0 to 3 mA.
The blue trace represents optical power, while the orange trace represents forward voltage.
The optical output remains very low at small drive currents. Near 0.8 mA, it begins increasing more noticeably, followed by an approximately linear rise over much of the remaining current range.
The voltage trace follows a different shape. It rises steeply at very low current and then increases more gradually as the drive current becomes larger.
Optical Power and Voltage Readings
The following values are estimated from the supplied graph.
Values are approximate readings from the supplied graph rather than raw measurement data.
The readings reveal three useful characteristics.
First, the estimated threshold current is around 0.8 mA.
Second, the optical output increases approximately linearly over the measured region above threshold.
Third, the graph does not show a clear thermal rollover within the displayed current range.
How the VCSEL L-I-V Curve Reveals Threshold Current
The threshold region can be identified by looking for a noticeable change in the slope of the optical power trace.
In the supplied graph, optical output stays near zero up to approximately 0.8 mA.
At 1.0 mA, optical power has increased to around 0.18 mW.
By 2.0 mA, it reaches approximately 1.15 mW.
This stronger increase indicates that the device has entered its lasing region.
The threshold is not determined by the first detectable photon. Weak spontaneous emission can exist below the lasing threshold.
Instead, engineers normally examine how the optical power changes with current and estimate the point where stimulated emission becomes dominant.
VCSEL Threshold Current Measurement
A VCSEL threshold current measurement helps identify the electrical current needed to begin sustained laser emission.
Threshold current is commonly written as Ith.
Its value depends on the semiconductor structure, operating temperature, and optical cavity characteristics.
A lower threshold may be useful in equipment with strict electrical power limits. Still, it is only one part of selecting a suitable VCSEL.
Estimating Threshold Current From the Graph
A common way to estimate threshold current is to extend the approximately linear portion of the optical power trace back toward zero output.
The supplied graph provides two convenient points:
At 2.0 mA, optical power is approximately 1.15 mW.
At 3.0 mA, optical power is approximately 2.10 mW.
The slope between these points is:
Slope efficiency = ΔP / ΔI
= (2.10 − 1.15) mW / (3.0 − 2.0) mA
= 0.95 W/A
For an approximately linear region above threshold:
P ≈ ηs × (I − Ith)
Where:
P is optical power.
ηs is slope efficiency.
I is drive current.
Ith is threshold current.
Rearranging:
Ith ≈ I − P / ηs
Using the reading at 2.0 mA:
Ith ≈ 2.0 − 1.15 / 0.95
Ith ≈ 0.79 mA
This gives an estimated threshold current of approximately 0.8 mA.
The value agrees with the visible knee of the optical trace.
Because the original numerical readings and measurement uncertainty are unavailable, this should be treated as an approximate graphical estimate.
Why Threshold Current Changes With Temperature
Threshold current is sensitive to temperature.
As the VCSEL operates, part of the electrical input becomes heat. The temperature of the active region can rise above that of the surrounding mount.
This affects optical gain, carrier behavior, and the relationship between the gain spectrum and cavity resonance.
For many VCSEL structures, threshold current increases as the operating temperature moves above a favorable temperature range.
The exact behavior depends on the device.
An Ith value measured at room temperature may not remain unchanged when the VCSEL is installed inside an enclosed optical assembly.
For dependable operation, threshold current should be evaluated under temperature conditions representative of the finished equipment.
VCSEL Optical Power vs Current
The relationship between drive current and emitted light is one of the most useful characteristics of a VCSEL.
The supplied graph provides a clear illustration of VCSEL optical power vs current.
After the estimated threshold near 0.8 mA, the optical output rises steadily.
At 1.5 mA, the output is approximately 0.68 mW.
Increasing current to 2.0 mA raises optical power to about 1.15 mW.
At 3.0 mA, the graph shows approximately 2.1 mW.
This behavior suggests an approximately linear light-current relationship over the plotted region above threshold.
Understanding Slope Efficiency
Slope efficiency describes how much extra optical power a VCSEL produces when additional current is supplied.
It is usually expressed in W/A.
Based on the readings between 2.0 mA and 3.0 mA, the estimated slope efficiency is:
0.95 W/A
This means that, within this portion of the measured curve, an additional 1 mA of current corresponds to approximately 0.95 mW of additional optical output.
A higher slope efficiency can be useful when the equipment needs more light without a large increase in current.
Still, slope efficiency should not be treated as constant under all operating conditions.
Temperature, current level, and changes in optical emission behavior can affect the slope.
Optical Power Is Not the Same as Electrical Efficiency
A VCSEL may produce more light as current rises, but the electrical input must also be considered.
Electrical input power is calculated as:
Electrical Input Power = Current × Forward Voltage
At 2.0 mA, the supplied graph shows:
Drive current: 2.0 mA
Forward voltage: approximately 1.88 V
Optical power: approximately 1.15 mW
The electrical input is:
2.0 mA × 1.88 V = 3.76 mW
Electrical-to-optical efficiency can then be estimated as:
Efficiency = Optical Power / Electrical Input Power × 100%
= 1.15 / 3.76 × 100%
≈ 30.6%
This gives an apparent electrical-to-optical efficiency of approximately 30.6% at 2.0 mA, based on the plotted optical power.
The calculation assumes that the optical reading represents the emitted power relevant to the efficiency calculation. If only part of the emitted beam was collected, the true wall-plug efficiency may differ.
Does Higher Drive Current Always Produce More Light?
Not indefinitely.
At higher current, electrical heating becomes increasingly important.
A VCSEL may initially produce more optical power with each additional milliampere, then begin to flatten as its junction temperature rises.
Eventually, the output may reach a peak and decrease.
This behavior is known as thermal rollover.
The supplied graph does not show clear thermal rollover up to 3.0 mA. Optical output continues to rise across the displayed range.
That does not mean rollover cannot occur at a higher current.
Measurements beyond the plotted range would be needed to identify the peak optical output and the onset of rollover.
How Temperature Affects VCSEL L-I-V Characteristics
Temperature plays an important role in VCSEL operation.
As the active region becomes warmer, several characteristics may change at the same time.
The lasing wavelength can shift, threshold current can vary, and optical slope efficiency may decrease.
These changes are particularly important in applications that require precise wavelength control.
Continuous-Wave and Pulsed Operation
VCSELs can operate under continuous-wave or pulsed electrical drive.
In continuous-wave operation, current flows continuously, generating a sustained thermal load.
Under pulsed operation, current is applied for limited periods.
Short pulses with a low duty cycle can reduce average heating, although the junction can still heat during each pulse.
Pulsed operation may allow higher peak optical output than continuous operation under otherwise similar thermal conditions.
The two drive conditions should not be treated as interchangeable.
When reading L-I-V specifications, check whether the measurements were taken under continuous or pulsed operation.
The drive conditions for the supplied graph are not stated, so the plotted values should not be presented as guaranteed continuous-wave ratings.
Why Packaging Influences Thermal Behavior
A VCSEL chip must transfer heat into its surrounding structure.
The heat path can include the semiconductor substrate, die-attach material, submount, package, and heatsink.
A suitable thermal arrangement helps keep the active region closer to the intended operating temperature.
Bare VCSEL die provide flexibility for direct mounting onto a selected substrate.
Packaged VCSELs offer more convenient handling and mechanical protection, but their thermal behavior depends on the package structure and mounting conditions.
This should be considered when moving from laboratory measurements to a finished optical assembly.
Common Sources of Unusual L-I-V Readings
Not every irregularity in an L-I-V trace comes from the VCSEL itself.
Electrical connections, optical alignment, temperature drift, and detector settings can change the recorded values.
A stable measurement arrangement is necessary when evaluating threshold current and optical efficiency.
Optical Collection and Detector Alignment
The measured optical power depends on how the emitted beam reaches the detector.
If the detector collects only part of the beam, changes in divergence or beam shape may affect the recorded reading.
The optical path should remain consistent throughout the current sweep.
An integrating sphere can be useful when measuring total emitted optical power.
The detector must also be suitable for the VCSEL wavelength and expected power range.
Electrical and Thermal Conditions
Current-source stability and electrical contact quality affect the readings.
A poor connection may cause unexpected voltage changes.
Temperature drift can also shift the apparent threshold and slope.
The following table shows several common observations.
An unusual curve is a reason to investigate, not immediate proof that the VCSEL is defective.
Selecting a VCSEL for Optical Sensing Applications
L-I-V characteristics help engineers select operating conditions, but optical power and threshold current are not the only considerations.
The required wavelength, spectral characteristics, temperature range, and packaging also matter.
This is especially important when a VCSEL is intended for atomic sensing or precision optical equipment.
VCSELs for Atomic and Quantum Sensing
Different sensing applications require different wavelengths.
Common examples include:
For atomic sensing, sufficient optical power is necessary, but the emission wavelength must also align with the intended atomic transition.
Current and temperature changes can shift the emitted wavelength.
As a result, an operating current chosen solely from the optical power trace may not provide the best spectral operating point.
Wavelength tuning, spectral characteristics, and polarization should be evaluated separately.
Bare VCSEL Die and Packaged VCSEL Options
VCSELs can be supplied in several physical formats.
Bare die are suitable for customers who need direct chip integration and control over their mounting arrangement.
TO-style packages provide defined electrical connections and mechanical protection.
Custom packaging may be preferred when equipment has special space, thermal, optical window, or material requirements.
Non-magnetic packaging is also relevant to certain magnetically sensitive applications.
Customized VCSEL Products From Semiatom GmbH
Semiatom GmbH manufactures VCSEL products for optical sensing, precision photonics, and related applications.
Our product portfolio covers multiple wavelength regions and packaging configurations, including bare VCSEL die, TO-style packages, and customized solutions.
For customers developing optical equipment, we can discuss VCSEL selection based on operating wavelength, optical power, drive current, thermal conditions, and package requirements.
The aim is to match the emitter to the equipment's actual working conditions rather than relying on one optical power value.
What Information Should You Share With a VCSEL Manufacturer?
When requesting a VCSEL for a new optical assembly, providing operating requirements early can make product selection more straightforward.
These details allow the electrical and optical requirements to be considered together.
What the L-I-V Curve Tells Us About VCSEL Operation
The supplied graph shows a VCSEL with an estimated threshold current near 0.8 mA and an approximately linear increase in optical output above threshold.
Optical power reaches around 1.15 mW at 2.0 mA and approximately 2.1 mW at 3.0 mA.
The estimated slope efficiency is approximately 0.95 W/A, while the apparent electrical-to-optical efficiency at 2.0 mA is around 30.6%.
No clear thermal rollover is visible within the plotted 0–3 mA range.
These values illustrate how L-I-V characteristics can help identify a suitable current range and estimate electrical power requirements.
For a finished product, the selected current should also account for temperature, wavelength stability, optical collection, and packaging.
Semiatom GmbH offers customizable VCSEL solutions for customers who need to balance optical output, electrical operation, and integration requirements.
Frequently Asked Questions
1. What is the threshold current shown in the supplied VCSEL graph?
The estimated threshold current is approximately 0.8 mA. The optical output begins increasing noticeably near this current, and a linear extrapolation from the higher-current readings gives a similar value.
2. What is the optical power at 2 mA?
The graph shows approximately 1.15 mW at a drive current of 2.0 mA. The corresponding forward voltage is approximately 1.88 V.
These are graphical estimates rather than exact source measurements.
3. How is VCSEL slope efficiency calculated?
Slope efficiency is the change in optical power divided by the change in drive current.
Using the readings at 2.0 mA and 3.0 mA gives an estimated value of 0.95 W/A.
4. Why does VCSEL optical power sometimes decrease at high current?
The decrease is commonly caused by thermal rollover.
As electrical input increases, the active region becomes warmer. Changes in gain and carrier behavior may eventually reduce optical output.
The supplied graph does not show clear rollover within its displayed current range.
5. Can L-I-V readings confirm the emission wavelength of a VCSEL?
No. L-I-V readings show optical power and forward voltage as functions of current.
Emission wavelength, spectral linewidth, and polarization require separate optical measurements.
These checks are particularly important for VCSELs used in atomic sensing and precision spectroscopy.
