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For many laser applications, such as marking for instance, it is necessary to control the quantity of energy with picking certain pulses. This can be achieved with a pulse picker and it is generally based on acousto-optic technology.
Теги: acousto-optic technology control of the laser energy quantity pulse picking акустооптический метод выборка оптических импульсов контроль лазерной энергии
Short and Ultrashort pulses are in most cases generated by a mode-locked laser in the form of a pulse train with a pulse repetition rate of 10–100 MHz or more. In many applications, such as marking for instance, it is necessary to control the quantity of energy by picking certain pulses from the laser outgoing pulse. This can be achieved with a pulse picker and it is generally based on acousto optic technology.
An acousto optic pulse picker is an electrically controlled optical switch used for extracting pulse (s) from a fast pulse train. There are no mechanical moving parts in such system and therefore the response time is in the ns range.
CONSECUTIVE PULSE EXTINCTION RATIO (CPER) AND RELATIVE EFFICIENCY (EFF)
In this paper, we will consider the case where a single pulse is extracted. In fact, two main parameters should be considered if not several pulses may be picked. These parameters include the Optical gate (Tw) usually provided by the user as well as Rise/Fall time of the pulse picker (Tr). They play an important role regarding the performance of the pulse picker namely:
• The Consecutive Pulse Extinction (CPER) which is defined as the attenuation value of the pulses next to the extracted one. It can be measured as an attenuation, or simply as a percentage of the non-picked pulse.
• The Relative Efficiency (EFF) which is the amplitude of the extracted pulse and measured as a percentage.
RISE/FALL TIME (Tr)
Ideally, the rise and fall time (Tr) should be small compared with the duration between two laser pulses. In other words, the pulse picker must be fast enough in order to rise between two laser pulses, and to go down fast enough before the next pulse (Fig.1). The table below shows the effect on the CPER and also the EFF while Tr is varied. Here, we assume that Tw is equal to the duration between two consecutives pulses T.
Note: When the rise time Tr is not small enough, the CPER is limited by Dynamic behaviour of the Pulse Picker but when rise time is fast enough, the CPER is limited by Static Extinction Ratio of the Pulse Picker (Static ER).
OPTICAL GATE (Tw)
On the other hand, the optical gate (Tw) should be wide enough in order to let the pulse picker reach its maximum efficiency. However, a large Tw implies also a degradation of the CPER. The table below show how the relative efficiency and the CPER is impacted when Tw is varied. Here, Tr = T/2
From the above results, we see that a small rise time implies a better relative efficiency as well as CPER. However, the choice of the optical gate duration should arise from a compromise between efficiency and CPER. The larger is the optical gate, the higher will be the efficiency of the pulse picker, but the lower will be the CPER. This case happens mainly when the rise time of the pulse picker is very small, and reaches the lower limit of the acousto-optic technology. Under these conditions, only the optical window duration can be used as a parameter in order to optimize CPER.
AA OPTO-ELECTRONICS PPK DRIVER
Even though the CPER and the relative efficiency have been optimised, the performance of the pulse picker may still be influenced by other factors. In fact, pulse to pulse instabilities appear in the extracted pulse(s) most particularly for high repetition rate lasers (Fig.2). This inconvenience is mainly due the non-synchronization of the input laser pulses with the optical gate.
To cope with the difficulties mentioned previously, AA opto-electronics designed a new driver (Fig.3): PPK and it has the following features:
• Input reference clock for synchronisation.
• High pulse to pulse stability.
• Built-in high accuracy internal generator.
• CPER Optimization.
• Optical gate width and delay adjustment.
• Picking ratio.
• Automatic reload of stored parameters.
• RS 232, USB, Bluetooth communication.
We have described the different parameters that need to be considered when performing an optimal pulse picking operation. Using a synchronized driver such as the PPK will definitely ensure an easier adjustment, a better optimization of the CPER, relative efficiency and above all, provide better results.
An acousto optic pulse picker is an electrically controlled optical switch used for extracting pulse (s) from a fast pulse train. There are no mechanical moving parts in such system and therefore the response time is in the ns range.
CONSECUTIVE PULSE EXTINCTION RATIO (CPER) AND RELATIVE EFFICIENCY (EFF)
In this paper, we will consider the case where a single pulse is extracted. In fact, two main parameters should be considered if not several pulses may be picked. These parameters include the Optical gate (Tw) usually provided by the user as well as Rise/Fall time of the pulse picker (Tr). They play an important role regarding the performance of the pulse picker namely:
• The Consecutive Pulse Extinction (CPER) which is defined as the attenuation value of the pulses next to the extracted one. It can be measured as an attenuation, or simply as a percentage of the non-picked pulse.
• The Relative Efficiency (EFF) which is the amplitude of the extracted pulse and measured as a percentage.
RISE/FALL TIME (Tr)
Ideally, the rise and fall time (Tr) should be small compared with the duration between two laser pulses. In other words, the pulse picker must be fast enough in order to rise between two laser pulses, and to go down fast enough before the next pulse (Fig.1). The table below shows the effect on the CPER and also the EFF while Tr is varied. Here, we assume that Tw is equal to the duration between two consecutives pulses T.
Note: When the rise time Tr is not small enough, the CPER is limited by Dynamic behaviour of the Pulse Picker but when rise time is fast enough, the CPER is limited by Static Extinction Ratio of the Pulse Picker (Static ER).
OPTICAL GATE (Tw)
On the other hand, the optical gate (Tw) should be wide enough in order to let the pulse picker reach its maximum efficiency. However, a large Tw implies also a degradation of the CPER. The table below show how the relative efficiency and the CPER is impacted when Tw is varied. Here, Tr = T/2
From the above results, we see that a small rise time implies a better relative efficiency as well as CPER. However, the choice of the optical gate duration should arise from a compromise between efficiency and CPER. The larger is the optical gate, the higher will be the efficiency of the pulse picker, but the lower will be the CPER. This case happens mainly when the rise time of the pulse picker is very small, and reaches the lower limit of the acousto-optic technology. Under these conditions, only the optical window duration can be used as a parameter in order to optimize CPER.
AA OPTO-ELECTRONICS PPK DRIVER
Even though the CPER and the relative efficiency have been optimised, the performance of the pulse picker may still be influenced by other factors. In fact, pulse to pulse instabilities appear in the extracted pulse(s) most particularly for high repetition rate lasers (Fig.2). This inconvenience is mainly due the non-synchronization of the input laser pulses with the optical gate.
To cope with the difficulties mentioned previously, AA opto-electronics designed a new driver (Fig.3): PPK and it has the following features:
• Input reference clock for synchronisation.
• High pulse to pulse stability.
• Built-in high accuracy internal generator.
• CPER Optimization.
• Optical gate width and delay adjustment.
• Picking ratio.
• Automatic reload of stored parameters.
• RS 232, USB, Bluetooth communication.
We have described the different parameters that need to be considered when performing an optimal pulse picking operation. Using a synchronized driver such as the PPK will definitely ensure an easier adjustment, a better optimization of the CPER, relative efficiency and above all, provide better results.
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